Odhanan said:
Furthermore, I have not seen, in my experience, the players actually slowing down the game because their abilities would be "per session". Mechanics being cleanly separated from the fiction, one does not come to mind as influencing the other in our minds.
It sounds more like it would change the focus of game-flow, but not necessarily break it.
I would hazard that in most games (including my own) 'per day' resource management, at least at lower levels, is a heavy influence on player actions. In my experience, it always has been. If the wizard only has two third-level slots and only one of them has a fireball, he's going to save it until he's sure it's going to get the best outing. The cleric will keep some healing spells in reserve and even the rogue and fighter are making judgment calls about when to use that healing potion.
When Complete Champion was announced with it's healing reserve feat (which I have not read yet), it raised quite a lot of discussion in my group. This had the dramatic ability to alter the cleric's role and healing as a factor in general. Let's face facts: players retire from the field whenever they are either tapped out of spells, hit points or both. Altering that dynamic can have far-reaching consequences....whether they would be good or bad, I don't know.
I'm thinking that maybe the problem with per-session or per-encounter abilities is that they change the dynamic in untold ways. If a fighter can pull out a single uber-strike once a combat, does that shift the 'cool things' balance? Are CRs now completely over-valued? Do mages get these abilities? What I am mostly concerned with is how this affects the whole group.
A few years back, one of my players, the cleric, staged a in-game protest during a fairly involved combat with some Frost Giants and winter wolves. The battle took place mostly under cover and involved lots of ranged combat. The rogue and cleric, feeling useless in this situation, sat down and began cooking soup. I resolved to try and not let that happen again. My main concern is that something like per-combat resets would heighten that problem, not remove it.