These rules are easily, and fairly, criticised on a number of grounds. Foremost among these criticisms is the rules' arbitrariness, encompassed in the notion that "the number of points given a player for a game session is a signal of how well the DM thinks the player did in the game". In other words, they're a method for rewarding whatever the individual DM's conception of "good role playing" entails. Favouritism, inconsistency, mistakes, unfairness, resentment - what could possibly go wrong? The worst element of all this is the idea that experience points should be awarded for meta concerns such as whether a player got involved properly, encouraged others, interfered too much, or acted like a "rules lawyer". Why it is the DM's business to passive-aggressively police the behaviour of the players through XP awards rather than just taking them to one side and saying, "Stop being an arsehole" is beyond me. Almost as bad is the idea of getting XP for achieving "story goals" - a rule that positively incentivises the absolute worst thing about bad gaming: railroading.
The beauty of XP for treasure is not just that it is simple but that it also tells you what "good role playing" is - getting treasure and surviving. It is what Ron Edwards might have called "coherent". This might not be to all tastes, but those other tastes are served by other games.
The 2nd edition way of awarding experience points was always unworkable in practice except in limited circumstances (like an adult DM with a group of youngish children), and it is mostly a historical curiosity now - or at least it should be. But it's possible that there is one bit worth rescuing. This is the class awards. Briefly summarised, these are:
- Fighters get XP for "defeating" enemies
- Priests get XP for successfully using their powers, casting spells to further their ethos, and making stuff
- Wizards get XP for casting spells to overcome "foes or problems", researching things and making stuff
- Rogues get XP for using their special abilities and, er, treasure