This mouth-music...is something I have trouble grokking.
I played my most improvisational and open-ended games in
3e. The robustness of the ruleset helped me do that, because I knew that wherever I would need a trampoline to spring me back up into the air again,
3e would have one.
I'm generally not a tremendous fan of DM Fiat, either. In fact, it's one of the least satisfying ways for something to happen in D&D, as far as I am concerned. It's necessary, always, especially in the corner cases, but as a "core mechanic," so to speak, I have a lot of resentment for it.
Sure, it's possible to play
4e and strip out the rules and just wing it, just like it's possible to play
3e that way, or any edition or game ever, anywhere. But I think the old school feel is more than just "make up the rules as you go along." I could be wrong.