Screen is fantastic.
unlike others, the iPhone 4 for me in the real world has BETTER reception that the 3G.
I can make and hold onto calls in my elevator in my condo that with my old phone would drop from floors 1-3. I can also get enough reception in our lecture hall that is underground and meant no service and only email (via wifi). Now I get just enough reception to make a calls and receive texts.
Build quality is exceptional. The iPhone 3G build quality was only OK IMHO.
Even though I like the iPhone 3G software, I hated the rounded form factor. It was hard for me to hold onto. The new form factor is not only a much higher build quality, but is easier to grasp.
The iPhone 4 is VERY snappy. The 3G was aways acceptable, but never snappy.
Battery life is phenomenal
Multitasking was worth the wait. I almost bailed to an Evo (but didn't because the build quality is junk when I tried one out) when I heard how apple was going to implement multitasking, but I must admit in practice it is very well done. Still a small bit of room for improvement, but overall excellent balance between battery life and function.
My only dislikes so far is the "silence" butter is hard to check when the phone is in my pocket.
The phone is just a bit to heavy.
Apple needs to implement wireless syncing for both the iPhone and iPad. Either mobile me needs a major overhaul or something, but getting documents and music onto the iPhone and iPad is still not good.