Fair enough.
I've run 3X on the fly many times and it has never been a problem. That said, I rarely run modules. I'm certain that I could not do that. But honestly, that doesn't have anything to do with game mechanics and everything to do with knowing how everything fits together. So if you *can* run 4E modules without any prep, then that just further reinforces my view of it.
Which is what, exactly? I tried to do 3e on the fly; and it failed. Not because of lack of story (the point I'm making with SR is that I don't have a problem coming up with story), but because of mechanical issues. Combat would too easily end up in TPK or cakewalk, with no obvious reason why. It ended up being very hard for me to adjust difficulty on the fly
and do so transparently to the players. That's the important part to me. really; that the players not be able to see that I'm fudging the numbers behind the screen.
I really can't point to a specific thing in 3E that made it feel like pushing through waist-deep water to run it; but that's what felt like, mentally. And laying a PC in that system wasn't much more fun. (Playing
with the system was kinda fun, building PCs, combos, etc.But not so much when the dice hit the table).
I don't get that feel from 4E. I've been playing a bit more than DMing, I wil admit. But last session I played in, except for some corner-case items, I played without reference to any book. That's a Good Thin(tm). My best gaming experiences have been when we haven't cracked a book at all in a session. I've had this happen in Rolemaster (GM was a system GOD), WHFRP, SR (on both sides of the screen there). Never happened, for me, in 3E.
I consider myself a moderately skilled GM. I am not a system master (I have generally expected players to know the subsystems for their characters more deeply than I do - though I will have a good overview). I know at least 3 people much better than I. But if I'm not "skilled" enough to run a game, there's 6 people who have to find something else to do on a Saturday night...
So from that POV, yes, 4E is doing it for me. It's enormously easier to run, and a good bit easier to play anything but a BDFighter. And the BDFighter got easier to play
effectively. Limiting mechanical options is not a Bad Thing(tm), especially for new players. One of the things that killed my last 3E eberron game was the hidden subtle complexities of a fighter. ("How much should I power attack for this round? Should I charge or move up? Wait - how much damage am I doing this round again?" I mean, there's some pitfalls in fighter powers - particularly with some powers adding the stat to damage and others not, but at least you don't have to have a statistics class.)