The d20 System is, in essence, the gribbly innards of third edition Dungeons and Dragons; the basic mechanics, shorn of all the setting elements. This means it has most of the classical trappings of the RPG - the idea that your character has statistics (which express how good they are at doing stuff), is a member of a class (which determines what skills they have easiest access to, i.e. what stuff they're best at), and has that many levels of experience (which make them better at doing stuff), and so many hit points (which are what they lose when something does stuff to them) and so on and so forth.
What it doesn't have, in and of itself, is much else. It's basically a series of mechanics, which are quite bare-bones but also quite granular, offering a detailed representation of exactly what your character is good at, and an attempt to customise the classes so that your twelfth level wizard is mechanically distinct from my twelfth level wizard.
The whole system turns around the rolling of a twenty-sided die to determine the success of a character's action - hence the name. Damn near everything in the game uses the same core mechanic; you roll a d20, add modifiers based on your character's statistics and skills, and compare that to a target number. That might be a Difficulty Check set by the GM (for those moments where you're doing stuff to the environment) or the Armour Class or opposed roll of a monster, character, other player character (for when you're doing stuff to things that don't want stuff done to them).
At its heart, it's consistent in that high numbers are always good, it's elegant in that everything runs off the same basic rule, and it's not terribly hard to grasp. The devil with d20 is in the details. So. Many. Details.
|Something beautiful and elegant is about to be ruined.|
See, your character has skills, and access to those skills is governed by the class your character belongs to. But someone pointed out that it's not strictly fair to say that their twelfth level wizard can't be good at swimming just because that's not a class skill for him. If he wants to make the effort, he should be able to do that. So there are cross-class skills, and rules restricting the rate of growth in them to keep the class skills attractive.
And then there are feats. Insofar as I can work out why feats exist, it goes something like this. At the start of yr. average D&D game, wizards are a bit poo. They're basically four squashy hit points in a dress with only two magic missiles per day and a big stick to defend themselves with. Fighters, meanwhile, have twice or thrice the hit points, bogart all the armour, and can use pretty much any weapon they like. So, wouldn't it be cool if my first level wizard could, I don't know, use a better weapon, like a crossbow or something, or maybe if he could wear armour and still cast spells, just to make things a bit more... what's the word... balanced, in those early stages? So I can pick a 'feat' for my wizard, when I first create him, and another one every few levels.
Hang on, though. After a certain point, fighters get really boring. The expression is 'linear fighter, quadratic wizard'. Fighters get better at fighting, to be sure, but wizards get to the point where they can call down a choking fog that kills pretty much everything in the room before the fighter's finished getting his sword out. So fighters get more feats than wizards do, essentially giving them some wild and crazy combat options like running through the room whacking EVERYONE with their sword at the speed of evil. Except... that puts us back into the 'fighters are cooler than wizards at low levels' corner... so what we do is, we make all the good fighter feats dependent on taking X, Y and Z boring fighter feats at low level, so that they can't get access to the good stuff until they've got to the levels where they need it.
All in the name of balance, of course. And all this stuff needs rules. Well... it sort of does. See, in a system without hard and fast rules for how you run through a room hitting everyone in it with a sword while trying not to get hit back, and do it all at the speed of evil, it's down to the GM to work out how that should be done (myself, I'd say a -1 hit penalty for every opponent after the first, and as soon as you miss one, you get hit, but that's me). And you can't always rely on the GM to do that, because some GMs are cruel and some are inexperienced and some just like having rules for things so that they don't have to do the developer's job for them. The d20 system is brilliant for that sort of GM. It basically insulates the players and GM from bad decisions, whether malicious or rooted in inexperience, because chances are, whatever it is you're trying to do, the system has a rule for it.
|YOU'RE PLAYING LET'S PRETEND WE'RE WIZARDS WRONG! IT SAYS IT RIGHT HERE!|
The downside, of course, is that it's a rules lawyer's dream. There's a rule for everything, and those rules exist to protect the GM from having to call everything and the players from bad GM calls, so why not insist on them being followed to the letter?
The other downside is that it's a pain in the ass to make character choices. Let me give you an example. I'm playing a Fighter, and as I've levelled this Fighter up, I've not rolled very well for his hit points, so he's looking a bit weedy for a supposed frontline combatant. So I decide to take the Toughness feat, which hands out three extra hit points per level, and beefs my Fighter up a bit. Fair enough. Except I only get so many feats to choose from, and I want to be contributing something at level 12, and I need to take these three boring filler feats to get that interesting one there, and can I afford to interrupt that just to get Toughness, or should I take Dodge and just soldier on? I'm sitting here fretting when I could just add another die's worth of hit points, another pip on my attack bonus, and trust the GM to come up with a ruling for whacking everyone in the room if I really want to.
To be fair, some versions of the d20 system are better for this than others. d20 Cthulhu's moderately restrained, for instance - the classes in that one only govern access to skills, and so there's no need for the elaborate system of checks, balances and filler in the feats system, so that's stripped down as well. Star Wars d20, meanwhile, is a bit of a bugger, with different creation mechanics for droid and organic characters, and all the paraphenalia of the Force to keep track of, and the added twist that you have a class that's basically a fighter and a wizard and so is basically cooler than everyone and so needs to be kept in check somehow so everyone else feels like they're useful... Oh, and if I didn't mention it before, the d20 system tended to attract writers with a serious case of logorrhea, and so everything has descriptive text sitting around it, of a quite long-winded nature.
The result is a system with a lot of stuff in it and a lot of words describing that stuff, and a basically elegant mechanic which is occluded by all this... bumpf. That said, if you like your RPGs to be balanced, and have consistent rules for everything, or if you like build optimisation and loophole-hunting, go to it and good luck - it's not the worst game out there and its heart is in the right place. If you'd rather make it up as you go along and get on with the story, and not be penalised in ten levels' time because you took Toughness instead of Dodge, try Swords and Wizardry instead.