Droid owner here, but ex iPhone owner when I lived in Europe (was not willing to switch to at&t crap service in new york)
For starters the physical keyboard on the Droid is unusable to me. It's absolute crap--the keys are flat, right next to each other, the D-pad gets in the way, and so does the top of the phone for the upper keys, I never use it.
the iPhone's virtual keyboard is really the gold standard, though the Droid's is perfectly fine, and the dictionary and spell check is superior.
On simulatinuous apps--meh, don't care dont use it. I loved the push notifications on Beejive when I used the iPhone, it works perfectly well and load time for Beejive on the 3Gs is super fast now. I didn't need it on the background.
On widgets, so far I only have 2 on weather and controlling wifi, bluetooth etc and it's great. Would be cool to have on the iPhone. I hope they implement it.
Open development? I don't really care.
Intercheangeable batteries? I don't care either, I don't have a spare and I'm not planning on getting one.
The Droid is a great phone, it does some things better than the iphone, some much much better like notifications, status bar, the iphone's screen is super crappy compared to the Droid's, native google voice etc....
But the iphone's ipod integration really is unmatched and it sucks now to carry two devices. The touch gestures are also I think much smoother on the iphone than the droid. That's software related so that's Google's problem to fix. Overall I am content with my Droid, but I do still think the iPhone is the best device out there and as soon as it comes to Verizon (and I assume there will be major improvements for the 4th generation) I will throw out the Droid!