Many ORPG games use the standard experience(XP and level model to reflect character growth and development. This, like hit points and armor class, is a holdover from MUDs (primarily DIKU)who, in turn, grafted it from Dungeons and Dragons. Generally, XP is earned by defeating enemies and greater levels unlock better abilities. However, it also leads to some ugly player behaviors. Two of the most common are "grinding" (the process of engaging in repetitive gameplay to gain advancement) and "camping" (waiting in one spot where a particular enemy spawns). It also often encourages content bypass (since players will flock to content with the highest benefit to cost ratio and ignore less beneficial content). None of these are generally considered particularly fun nor desirable.
In the MUD dev archives I came across an interesting alternative that I am strongly considering. Basically, it works like this. Characters have ratings for skill groups (craft, combat, lore,etc.) as well as individual skills. By completing tasks, they gain knowledge that makes advancement in that particular skill easier. However, to make significant gains, they must push their skills and attempt increasingly more difficult tasks. Completing tasks also makes it easier to advance in related skills.
For example, Bob is a baker of Novice skill. By successfully baking a cake (a fair accomplishment for his skill), he gains some points that can be used to advance. Since he used a craft skill, these points are tied to craft. So, it might not be enough to raise his brawling abilities (unless, of course he had also been using that skill), but it probably is enough to bump a craft skill up a bit, especially cooking. Additionally, since he has demonstrated an accomplishment of skill, his cooking "soft cap" would loosen a bit. If he bakes another cake, he will gain a few more points, but would not gain the cap benefit again, because he has already accomplished this and now needs to do something more difficult.
Let's say he instead was just baking cookies repeatedly. Cookies are easier, so he gains fewer points. Additionally, since he hasn't really accomplished anything, he is going to be stuck at Novice unless he tries something harder. And, after a while, baking more cookies won't get him any more points (though he will be very good at making cookies!).
As far as I can tell, this seems to reward players for attempting tasks in areas they wish to advance while allowing some wiggle room for a little general advancement at lower difficulty without devolving to constant repetition. It allows some cross training, but since skills inevitably require some practice to use, you don't get the master baker suddenly turning into a dragon slayer.
I also envision tutors who can help teach the basics of skills by assigning tasks and providing instruction.