Seriously, experience points and character classes were the two main reasons that I turned to RQ (now BRP) in the first place.
Classes: I want these to arise from the game world, not from the rule book. So, for instance, the worshippers of the God of the Undead gain abilities to speak to undead, control undead, and ultimately cheat Death as they sacrifice POW and take higher oaths. Similarly, those who have studied with the monks at Mountains Meet have learned certain bare handed attack techniques, and the ability to pass without trace. All without formal character classes. This may be a little bit of semantics, but to me it's important that there is an in-game source or authority for character abilities.
Experience: As a GM I hate tracking and awarding experience. And I just hate the inevitable "what counts to get you experience" discussions. The "Use it and advance" approach of BRP is very simple and elegant.
This is all purely my preference. No reason not to graft a class or experience system on BRP, but surely there is an easier way to get where you want for your game? :confused:
Bathalians, the newest UberVillians!