Here's something that annoys me greatly about traditional RPG systems, something that, unfortunately, is seen as inherent to the very core of role playing games, and that something is the concept of character classes and experience levels. I do realize that RPGs have to take certain liberties with "realism" in order to offer you, the player, comprehensible rules of the game universe, which is how we get hit points as a stand in for a character's health or skill ranks that quantify the character's abilities. However, I believe that the traditional class and experience systems are far more restrictive than they need to be.
For those of you who don't know, a "class" is the RPG term for a character's profession. One way to look at it is that a class is basically a pre-packaged bundle of skills, feats and special abilities that define the character's role in the adventure. Fighters fight, wizards cast spells, bards do bard stuff, rogues do rogue stuff. The problem with this is, though, that your character usually ends up defined entirely by their class. You're not Lord Gabriel of Asken, you're The Fighter. You're not Moonslip the Orphan, you're The Thief. Players are discouraged from differentiating their cleric from the bajillion other clerics that came before. The cause of this problem is the way in which the class system restricts what skills is any given class allowed to possess, either by making investing in cross-class skills expensive or outright impossible. If you're lucky, your system will allow the thief instruct the ranger on how to pick pockets or let the monk train the mage in advanced staff combat skills. If you're not, well, better hope you like having no identity.
I have encountered a particularly nasty case of this in the Czech knock-off version of Dungeons & Dragons called Dračí Doupě ("Dragon's Lair"). In this system, a character's abilities are given entirely by their class, of which there are five (each of which then branches out into two different prestige classes at level 6). The rules spend a lot of time outlining what each class can or can't do, including what equipment they're allowed to use, but then the DM's Handbook contains a section that essentially says "well, of course these things aren't forbidden per se, you just have to think real hard about why they're said so and rule accordingly". In other words, the system forbids perfectly rational actions (such as a ranger using a stolen heavy weapon when breaking out of jail or a wizard wearing armor) for no reason other than ballancing issues and then provides the DM with no way to resolve these issues apart from "use your best judgment".
I have mentioned the traditional experience system, too. Unlike the class system, this one doesn't really irk me all that much and I can see why it's so popular, after all, a character's level is a clear indicator of how strong that character is. Also, it provides an easy way to simulate the growth of a character's capabilities. My only problem with it is that it's really not all that important and it allows the flaws of the class system to exist. This is especially evident in systems that allow players to make multi-class characters, as their experience levels count towards their classes, not the character as a whole. I like to say that the levelling system solves problems that would otherwise not even exist.
Saying all this, I believe it's no surprise that I have decided not to implement a class or levelling system in my homebrew, instead opting to go with an alternate system that has appeared in parts in other games. The cornerstone of this system consists of what I call "advancement points" that are either "bought" for collected experience (the homebrew specifically set the price of one advancement point as 100 experience points) or awarded directly by the GM. These points are then used to, well, advance the character, mainly by increasing skill ranks, but also by buying feats or improving hit points (if your system has those things). I'll be the first to admit that this system is far from perfect - it is rather simplistic, even though its implementation requires balancing out the costs and the rate at which the points are awarded - but I like the way it smooths out character progression.