Okay, Harkins wants me to start posting, so here are some of my thoughts on a handful of the more popular mechanics out there. Don't be offended if I don't like the mechanic that you like or I like one that you hate. My goal with this post is to get people talking about what they want from a mechanic so we can all enjoy games a lot more.
We all know what game mechanics are...the system of rules that turn a fantasy world into a game world (i.e., they let you create a character and participate). Since the mechanic is the protocol you use to create your character (which is, for all intents and purposes your avatar) and make that character interact with the game world, it can go a long way toward helping or hurting your gaming experience.
So, how does the mechanic help or hurt your gaming experience? It alters the feel of the game world. For instance, if the mechanic has watered down difficulty numbers, the game won't feel realistic. If the mechanic has magnified difficulty numbers, the game will feel grittier and more realistic. If the game doesn't have a fear system, your character will seem more heroic. If the game doesn't have a sanity system, your character will have nerves of steel. If the game doesn't have a weather system, weather will have no effect on the game world. The examples are endless.
I'm not going to say that a mechanic needs to have every conceivable system (although some mechanics try to go that route), because that's not the case. All a mechanic needs is the ability to create suspension of disbelief. A good mechanic should leave you convinced that things in the game world will happen very much the same way as they happen in real life. It helps you immerse yourself in the storyline. There are always gaps...ways in which the mechanic lets you down or goes against what's intuitive, but a good GM can go a long way toward smoothing out those rough edges (since the GM is the final arbiter of all mechanics, and can create or disallow rules at will). So, having gone over the basics of what I'm going to talk about, let me give you my thoughts on a few popular mechanics.
D20: It's hard to make any blanket judgments about D20. It's become so pervasive in paper and pencil gaming that saying you don't like D20 is roughly analogous to saying you don't like to eat mammals. With the sheer number of mutant varieties of D20 and optional rules variants, it's a good bet that there's some version of the D20 system out there that you would find acceptable. I'm not saying that it's impossible for there to be "vegetarian" types who absolutely cannot accept a d20-based mechanic, but usually that kind of blanket rejection of d20 is more about non-mechanical concerns (e.g., "I hate Wizards of the Coast, so I must make a big show of disliking their game system and all its derivatives lest I accidentally support their evil empire").
D20 is principally concerned with physical effects, especially combat and magic. I'm not going to say this is a bad thing, but it limits your options for roleplaying. It can work if you have a versatile GM and at least one player who wants to talk to NPCs. However, there's no question that the system got its start in tabletop wargaming.
The big mechanical problems I see with D20 are hitpoints and classes/levels.
Hitpoints: Hitpoints are usually taken to be a character's "health" (if you think in terms of videogames, like I do half the time). However, hitpoints are a lot more abstract than that. The first thing you'll notice about hitpoints is that they increase over time. That really isn't the case in reality. If you listen to the great proponents of D20, the increase of hitpoints over time allegedly represents your character's increasing experience and ability to avoid damage in the first place. When you get down to it, this really doesn't make sense...why not increase the AC instead? Is a middle-aged man really capable of taking twelve arrows to the chest when a twenty year old can barely survive two?
Now don't get me wrong...I'm not saying that it doesn't make sense to have a hitpoint system in your mechanic. If you're playing a heroic game, it makes perfect sense. I think what D20 fans mean to say is that the increase in hitpoints reflects the character becoming increasingly important in the game world. For example, if you're reading Lord of the Rings, it wouldn't fit in with the game world if Aragorn slipped on a banana peel, fell down the stairs, and broke his neck. So, why not just give more dramatically important characters the ability to soak more damage, even if it isn't realistic? Since realistically vulnerable heroes would hurt the feel of the game world more than the loss of realism from having the hero get smashed in the face with a sword and live, you can justify using hitpoints. And again, a good GM can help you gloss over the lack of realism by artfully describing solid hits as glancing blows, shallow cuts, or stunning blows that got blocked by armor, etc.
However, if you want to run a gritty, realistic campaign that captures the uncertainty and danger of real steel on steel combat, hitpoints will just frustrate you. Mid-level players will get in fights with elephants or whole units of cavalry, and generally flaunt their increasing ability to soak damage. Predictably, bad GMing doesn't help, and players will come off looking like Marv from Sin City at best or the Black Knight from Monty Python at worst. But again, there's nothing wrong with that if that's the feel you're looking for, and it very well could be (some friends of mine and I made and played a Warhammer 40K roleplaying game from D20 where that was the perfect feeling to capture the game world, and we all had ridiculously powerful characters).
Classes/levels: Like hitpoints, classes/levels are something you'll love or hate depending on the feel you're looking for. Fundamentally, these are part of a very black and white game world where your interests and abilities are tightly restricted to those that are stereotypical to your profession (e.g., fighter, rogue, ranger, etc.). Let me just say right now that I can anticipate a lot of responses to what I've said so far (which is pretty clearly biased against classes/levels).
People will say that classes are only umbrella classifications for a whole spectrum of subclasses and different distributions of feats and skills. A fighter could be a pirate, a mercenary, a castle guard, etc. You get the idea. Well, what that's really saying is that you're capable of making two characters that are fighters that are different. My opinion is that a mechanic owes you that level of customization from the getgo anyway, so it's stupid to try to pretend like it's a feature.
As far as different sub-classes go, any cursory look at d20-based systems will reveal a proliferation of custom classes and/or prestige classes. There are more of these every month. Personally, I take their existence to be proof that all d20 players still struggle with and dislike the restrictions of classes/levels. And to be honest, the generalization of classes (e.g., strong, tough, fast, etc. rather than fighter, ranger, etc.) that appeared with d20 modern leads me to believe that the concept of classes/levels is finally being recognized as a failure. I also think attempts to give players the formula to balance the classes (as in the D20 Call of Cthulhu game) and tell them to make class design part of character creation are a similar admission of the stupidity of classes/levels.
There may be cases where you want to use classes/levels for whatever reason, but I honestly think the reason they're so popular is that the D20 mechanic is so well known. As I say, even D20 enthusiasts are constantly trying to find ways to tweak or cicumvent the class/level system. Feel free to debate me on this, but I would always opt in favor of a system that allows a totally free-formed buy of all attributes and skills over time (7th Sea, Legend of the Five Rings, D6, and GURPS are all good examples). Although, to be perfectly honest, a purist will tell you that mechanical differences between characters (e.g. skills, feats, special abilities, etc.) are irrelevant, and it's the roleplaying of a character by a player that really customizes them. And if you know a player that actually believes this, make sure you invite them to all your games.
Regarding the actual use of a 20-sided die, the probabilities behind all these mechanics are a complex enough issue that I'll set them aside for a later, separate debate over mechanics. On to another major mechanic.
White Wolf: White Wolf's D10 system is essentially the ultimate roleplayer's system. When you're dealing with abilities that run on a 1-5 scale, you can tell you're definitely in Kabuki theater mode. It's a roleplaying intensive system, and physical interaction definitely has a "do we have to?" feeling to it. It almost moves in the exact opposite direction as D20 (which, as I've said, is the classic hack and cast, dice-intensive mechanic).
Combat in this system is probably best run with a Shakespearean flavor, like the lightsaber duels in the original Star Wars movies...more about soliloquys than combat (e.g., trade some blows, banter a little, trade some more blows, banter a little bit). Also, the tiny scale for both attributes and skills (1-5 in both cases) makes for characters that are writ large, but either not particularly believable or not very powerful.
When you take the system in combination with White Wolf's poorly thought out game world (the World of Darkness) this can be a GM intensive system. At least, that's how it is if you do things right and run a serious, complex game. If you have a shallow, adolescent, power fantasy game (e.g., Lord Dark and Night Ass are locked in a struggle for who gets to do nothing with his secret control of the world while assorted female characters walk around making seduction rolls), White Wolf's mechanic and game world are totally sufficient without much, if any, effort on the part of the GM.
Before I go off on a rant about game world design, which is inevitable when I talk about White Wolf, let me just say that this is a barebones system in terms of physical interaction. In the interest of fairness, I'll say that White Wolf's system has a more complex mechanic for social interaction than D20, which is more evidence of the system's bias in favor of roleplaying over violence. To put it as simply as possible, D20 is about violence...White Wolf is about threats of violence.
Another place White Wolf's system does well is character design. They have a sort of middle path between totally free-formed characters and a class system. Typically, a White Wolf game will break down characters into a handful of categorizations (e.g., clan, tribe, etc.) and then give you free reign in skill selection and advantages/disadvantages (which puts it head and shoulders above the extremely narrow feat and skill customization of D20).
GURPS: Unfortunately, I don't have enough experience playing or GMing for GURPS to properly evaluate it. What I've seen, I've liked.
Shadowrun: For my own purposes, the most impressive system I've seen is whatever 4th edition Shadowrun is working off. It's a D6 based system with character free-forming. They eliminate a lot of foolishness by saying that the level of zero in a skill is equal to "the average man on the street." This is definitely open to interpretation, but I think it produces better characters. Also, the clear delineation of what each level of skill constitutes is a nice feature (basically, a list of examples of who has a skill level of 1, skill level of 2, etc.). That, in combination with a similar scale for attribute levels, makes it easy to see where your character stands in the world (and ensure that your character is the character you thought you were making). The system's big weakness is an oversimplified roleplaying mechanic, but it's still about as complex as White Wolf's, maybe moreso.
In any case, I'm just trying to get some discussion going about mechanics. And again, this doesn't even discuss the relative merits of different dice, or other...less traditional methods of randomizing results (like rock, paper, scissors while dressed as a vampire).