Here is what I do not understand: a student in high school can only participate a very limited number of times verbally in class because there are other students. In D&D there is a limited amount of magic items that need to be tried out. How does any DM calculate the involvement in such things into XP values given out after every session. My guess is that he/she would only be able to do this in very broad strokes, which does not seem to serve the purpose of XP as a measure of involvement of a player in the game.
In a game like 5e, the death of a PC can be caused by one stupid roll of the die, especially in earlier levels. I do not understand why a player to whom the survival of a precious PC is paramount should be penalized by recieving less XP as somebody with a more reckless behaviour.
I just need to point out that characterizing not awarding xp for things you didn't do as 'punishment' is one of the big divides here. You see
withholding xp as a punishment, whereas I (and I suspect Lanefan, and others) see
awarding xp as a reward. From one perspective, everyone is
entitled to the same xp; from the other, nobody is, and it must be earned actively.
As usual, this divide boils down to a difference in playstyles. Neither is objectively right or wrong, but one might be wrong or right
for a given group.
The issue of how you play your PC is really not an issue that should be handled with XP. Instead, it something outside of game mechanics and should be discussed to form a common basis on what the behaviour in the group as a whole should be.
Well, partially, but encouraging certain behaviors is exactly what xp does. Players are far more likely to do things that get xp rewards. They tend to look for ways to get more xp, whether by fighting bigger, badder monsters in a system where xp come from monsters, by accumulating treasure in an xp-for-gp system, by working towards completing the story or plot they're running through in a system that awards xp for advancing the story, etc.
I'd say that (for those of us using xp) it's well worth looking at what activities provide xp, and if we want to encourage a certain playstyle, to bear in mind that xp are a great tool for motivating players toward that playstyle. Skipping xp and awarding levels at milestones falls under the same principle; it's pushing the players to follow the story or hit those milestones (whatever they might be).