For me, the form factor. It's a personal preference thing, but I was never comfortable holding a 4. I know it's thinner and lighter, but it felt thicker and heavier. Also didn't slip into the pocket of my jeans with the careless of abandon of the 3GS; always felt like I'd snag an edge and drop it; almost did a few times. And the edges made it uncomfortable to hold. I know, buy a case. But I prefer not to bulk up the phone with a case if I can avoid it.
Oh, also I was able to get it in white day one, not wait almost a year for a white one -- I gave up waiting in October of last year and bought a black 4.