1E and 0E gave out XP mainly for loot and a little bit for killing things. 2E defaulted to ad hoc XP (the DM makes up the award based on how quickly they feel the players should be advancing and how well they feel the players role-played), with optional rules for quest-completion XP ("saving the baron's daughter is worth 1,500 XP") and class-specific XP (thieves get 100 XP for picking a lock, mages get 500 × spell level XP for inventing a new spell, etc.). In fact 2E devoted one small paragraph to XP-for-treasure, saying that DMs could optionally do that instead, but cautioning against it since it "tempted DMs to hand out too much treasure." (The text of 2E could sometimes feel very passive-aggressive about the way 1E did things.)
Well that wasn't how I remembered 2E working so I just went back over it. Re-reading the 2E DMG, saying that it defaults to ad hoc xp awards is... inaccurate. It starts by talking about giving xp for fun, survival, improvement and story, but without presenting any numbers. When it comes to advancement the classes still use the same xp charts that they did in 1E. Those awards still come in numbered amounts. So, actually adding up points means you still start by adding up xp for monsters killed (and the values for that are about the same as they were for 1E except that 2E smartly eliminated a lot of the pointlessly nitpicky calculations for that which 1E had). Story award value was then constrained by that number from the monster tally. Essentially, instead of giving xp for gp, 2E said to assign a value you think is fair based on the completion of the story/adventure/whatever - and that the amount of that award should not be higher than the amount for killing the monsters, nor should the story award be higher than 1/10th the amount that a character needs to gain a level. So if you need 100,000 xp to advance to the next level and you gain 15,000 xp for killing monsters on a big adventure, the story award for that adventure (which would normally then get as high as 15,000) should actually not exceed 10,000 xp. Also, it says that the story award is most importantly for use in simply pacing advancement where the DM thinks it should be.
The inference that should probably be drawn from that is that ad hoc
portion of 2E's xp awards is less about what actually happens on an adventure than it is about ensuring that PC's continue to gain xp and advance
regardless of what happens on an adventure, or that if they kill a LOT of stuff that they not advance TOO fast, so give them a smaller story award. So, if it should happen that you do no combat at all, being limited to 1/10 what you need to advance to the next level you'd then need to do 10 NON-combat adventures. If you matched monster-killing xp with story xp every adventure you'd still need 5 adventures per level. Only if you're getting more xp killing monsters than you are in maximizing your story award would you advance faster than that.
It also says that any award for survival should actually be very small and such an award should even be reserved only for very special occasions. I'd note that the way the survival award is talked about anyway makes it not a group award but an individual award and they don't specify that. Otherwise, your PC's award for survival gets lumped in with all the other xp and then divided equally among the group.
Replacing 1E's xp-for-gold with other things is not a bad idea at all. 1E still had a VERY narrow and actually outdated view of what the game actually was and how it should be played. It was actually already being played quite differently than it had been in OD&D and ideas about xp needed to expand - but 2E is NOT entirely ad hoc. Its gameplay is still solidly rooted in killing things and taking their stuff - it's just that the stuff is mostly now its own reward, so to maintain the SAME pace of advancement and make up the rest of that xp that 1E handed out they said:
"For something like half the xp award just... MAKE UP what it's for rather than saying it's just for the gold. The
pacing of advancement is as important a thing as much as what you're actually doing. So start with the same numbers as 1E had for killing things, and then just add some more to make up for the removal of xp-for-gold. Call it a "fun" reward, or "story" or "improvement" or whatever. Oh, but don't make it TOO much. Still gotta keep the PC's actually adventuring for that xp."
2E made changes, even improvements to xp awards, but they weren't revolutionary, just evolutionary. Just don't get me started on the disaster that was 2E's optional individual award system to show that they were not entirely on top of their game in designing xp systems.
