For me, having individual R, G and B curves was a real draw but I must say I've hardly used it so far. I might get me some film simulations and see how I like that, I was really fond of those when all I had was the GIMP.
The range of highlight / shadow slider adjustments has apparently been greatly improved, although I still find myself maxing out the highlight reduction slider on a fairly regular basis - I don't know how much less capable LR3 was in this regard since I didn't use its sliders enough when I had the 30-day trial.
I think I was happier with LR3's auto exposure tool than LR4's, but thankfully it's really easy to apply the sliders manually - oh, and the sliders' organisation and differentiation has become more logical
other than that, I'm still only getting to know LR4 so I haven't used anywhere near all of its functions yet, but if you don't really need any of the new functions, LR3 remains an incredibly powerful program...