What I can do with my N1 (the same would apply to a Droid) that an iPhone can't:
1) Download files in the background with Dropbox, Tube Downloader, the market, upload a batch of photos through the gallery, or... all four at once! All this while streaming music, viewing google maps, and having several windows open in the browser. No, the phone doesn't slow down. Oh, yeah, and I can download things directly through a browser. Like on a real computer. No, you can't click links and download things on the iPhone's browser.
2) Use it as a giant USB stick. iTunes? Piss on it. I plug my phone in. Mount, drag, drop, done. No bloatware needed. No fumbling around with an adapter. Standard ports are beautiful things.
3) Oops. My internet connection is down. Oh well, toggle my phone's data connection on, plug it in, tether. Voila, my PC's now online again.
4) I'd like to try out some alternative browsers. I go to the market, download Dolphin/Steel/Opera Mini and install. On the iPhone, you're stuck with Safari mobile forever. Sorry, Steve Jobs doesn't believe in free will and choices.
5) Don't like the default look of Android? Get a home replacement and make your homescreens look like webOS, or Sense-skinned WinMo, or whatever. Have your screen transition turn into a 3D cube. Enable rotation for your launcher. Theme everything blue. Theme everything pink. There's something for everybody (within limits; if you want more, you'd have to root--but even without rooting, an Android phone's customizability crushes the iPhone's one-icon-grid-for-all "I'm a sheep, baa, baa" look to a pulp).
There's plenty more, but these are some of the more fundamental, basic things.