And 1 XP was awarded for keeping 1 GP worth of treasure found during the adventure. It's difficult to overstate just how important that is in driving play — with that rule in place, you quite literally never have to "dangle hooks" in front of the players. They'll seek out dungeons on their own, because they want treasure, because they want to level up.
A thing that is sometimes overlooked- and I didn't read the entire thread, so someone else may have already brought this up in more detail- is that if you use xp, you don't have to use it as written. Instead of just giving xp for defeating foes, you can give it for treasure. You can give it for roleplaying. You can give it for achieving goals, personal or party, or for whatever you want to motivate your party with. I love xp- I love the granularity of it and tracking it, and how it allows different characters to swap in and out of a group without artificially tying their advancement together, and I love how it enables mixed level play (by letting lower level characters catch up with their higher level pals).
I'm currently running a group using what I call the "ale and whores" xp system. Basically, the only way you get xp is by burning money on stuff nobody gets any real benefits from- thus the name. This pushes the characters toward a playstyle that reflects old Conan stories and the like- find adventure, get rich, party down with some Stygian lotus until you punch a horse in the face, get broke, find a new adventure. It also encourages things like hocking your armor for a few extra gp to drink away if you are just a bit short.
On the other hand, in the past, I used a system where you got half the normal xp for overcoming threats and then, at the end of each session, you could get xp for role-playing. For a while I had a "four categories" system- you could earn xp for role-playing your class, race, alignment, and personal traits. At a different point I had a system where everyone picked ten personality traits, with the option to add more as they leveled up, and at the end of the session they got xp for each trait they roleplayed (to a max of ten) during the session. These traits were completely player-defined. "I like dogs. I'm scared of spiders. I like the color blue. I prefer to be barefoot." Whatever.
Another thing I liked about this was being able to tailor my group's advancement rate. So for instance, I ran my "four categories" system in 3e, where you needed 1,000 xp to hit second level. I decided that if a group didn't do anything dangerous, but did do consistently good role-playing, I would like them to hit 2nd level in ten sessions- so rp xp should come out to a potential 100 per session, so 25 per category at first level. (And given the way the xp charts worked in 3e, it was easy to scale this up.) Under my ten traits system, each trait could earn you 10 xp per level per session and you would get the same (or a similar) advancement rate.
(This rather slow advancement rate also served to incentivize risky play for those quicker, easier combat xp. The combination of combat xp and rp xp tended to advance the group slightly slower than the standard just-challenge-xp system, but one could easily adjust to taste.)
Anyway, point is, xp can be versatile and serve a lot of different purposes. I think it's biggest advantage is decoupling the advancement of pcs from each other- which I know is a controversial stance these days.