With USR, I wanted to make character advancement a little different from the norm. While it still sticks with experience points, the approach isn't what you get in, say, D&D.
For instance, a character's main source of experience doesn't come from killing monsters. I know that GMs are often encouraged to offer points for roleplaying etc, but in games like D&D combat is where a bit chunk of experience comes from. I don't mind this at all, but I don't find it entirely logical. Why would killing a group of orcs make the Bard more charismatic or the Wizard more intelligent? I suppose you could say that the types of attacks they make, such as Bards using charisma-based attacks, will contribute to their experience, but I'm not entirely convinced. In USR, however, characters only gain experience points through great roleplay, teamwork and acting intuitively. I'm not saying this is the perfect way to go about experience, but it's a different one that feels more logical.
Moreover, advancement in USR doesn't follow the usual conventions of increasing an attribute or two when a character gains a level. I don't believe that you can get more intelligent. You can learn new things and maybe try exercising your memory, but on the whole your intelligence doesn't increase. You might disagree, but I don't see it. However, you can train yourself to be stronger, faster and fitter, so that part makes sense (kind of). In USR characters don't increase their die-size when they go up a level. Someone with Wits d8 always stays that way. Instead, either they increase a known specialism or learn a new one based on their past performance. They don't get smarter, but they can learn more about something specific or something they already know. If they were able to increase their Wits to d10 then that implies they've somehow just got more intelligent, which never happens, but it's perfectly logical that someone who has Geology +2 can learn more about the subject to become Geology +3.
I don't think that my way is the best way, but I wanted USR to present something a little different that reflects reality a little bit more than some games when it comes to advancement.
You can download USR for free from RPGNow.