The default in most people’s minds is leveling. A player accumulates experiences points doing whatever it is that the game wants to reward, and when she hits a particular break-point the character suddenly increases in abilities across their entire spec.  The important thing here is to understand what sort of behavior players are rewarded for doing most often.

Alternate Systems could include


While I'm certain there are other ways to reward players... my systems utilize a few of these... specifically,  the 'Standard XP', but also 'Advancement by failure', 'Milestone', and 'Advancement as Currency'.