It seems like lately I've been reading some blog posts blasting D&D for not having rules for earning XP beyond killing things and taking their stuff (don't ask me to point out specific blog posts, I can't do it, so you'll have to trust me on this one).
Perhaps these bloggers were referring to OD&D or Moldvay/Cook/ Marsh BX. Why do I say that?
Because the Rules Cyclopedia (a consolidation of Mentzer's BECMI) has an experience chapter (Chapter 10) that breaks down how characters earn experience thusly:
1. By Role-Playing Well
2. By Achieving Party Goals
3. By Defeating Monsters and Opponents
4. By Acquiring Treasure
5. By Performing Exceptional Actions
So, as you can clearly see, earning XP from killing monsters and getting treasure are numbers 3 and 4 on the Rules Cyclopedia list! The chapter goes on to give the details on how exactly XP of all types is actually determined (I can't get into it now, sorta busy with work and all).
Thinking back through the misty fog of my memory, I seem to recally that, though I never played using the RC rules (again, being firmly an AD&D kid) I nonetheless read the Cyclopedia and mined it for information. This includes the experience chapter.
My love for the RC grows and grows...