The difference is that 4E is built to encourage, and to a certain degree, require, the kind of thinking on the player's parts that drives most on the fly decisions, while 3.x/PF really isn't.
4E encourages specific tactical actions, not on the fly, impromptu actions.
You have to really work to incorporate aspects of the environment to have an effect and get players to not look down their list of powers, where their decision is "do I burn my daily now or just go with this encounter power?" And, eventually it comes down to "spam at-will".
Now, some DMs have tried to mitigate this (with liberal use of DMG Page 42), but in my experience, that is few and far between. But, when I played 3.5 (years ago), I found there to be a lot more "on the fly" sort of creative approaches to things.
And, in my Sunday B/X game, on the fly is pretty much _all_ we do.