I love 4e - been having a blast.
With that said, to answer your question...
What has been lost is The Mystery.
I have explained this to a few people as such...
Before 3e (so I guess I am saying the mystery was lost then) if a player swung his hammer and i said, "it stops inches from the troll and slams into some force, at which point your hammer disappears", the player would respond with something like "whoa, crazy - I wonder what that was about"... but in 4e, the response is "umm, what? there is no power that takes my weapon away".
Of course as a DM I can say, "well, it does" and sure, my player will say, "oh, ok, I get it, cool!" but the mystery still gone - it is very clear I stepped in and applied something outside of the rules to adjust something for the sake of the story.
I think, at least in my experience, this was easier back in the day because there were simply so much info out there with rules etc that no one could possibly know it all (well), and so these things would get chalked up to "well, i haven't read everything, so what's happening here is something i have not seen yet" - whereas 4e is very clear and puts so much of it in the hands of the player that it is really obvious when the DM steps outside the box. Again, this is in my experience. I know there are players out there who (back then) knew it inside and out and could call a DM on any of those things - I just never saw it personally.
My brother summed it up once, as we were in agreement, by saying he felt 4e "took away his creative license". It doesn't mean he doesn't like the system (he is running a 4e campaign and having a blast), but that the mystery is what he felt was missing as well.
With that said, if killing the mystery (as I apply meaning to this term here) was required in order to achieve the balance that 4e has, that's fine and I'm on board - balance is one of my favorite things about 4e and so I will give up some of that mystery. As it stands, I tend to start my 4e campaigns with the disclaimer "this campaign is more about the story than the rules - if you see something occur that might fall outside the rules, please keep that in mind" etc.
Again, all of this is from my own experiences - Myself and others who played with me back in those early editions were pretty inexperienced. It could be that if we were more experienced, we might have known all the rules much better (and recall them all easier) - and we could have felt the same then as we do now with regards to a lack of mystery - but I'm not sure.