I suppose first you have to ask, what's so great about the iPhone?
I think the overall answer is that the iPhone is a smartphone that's easy and fun to use. As I keep saying, it's the smartphone for people who didn't know they needed one. (The rest of us had smartphones for years, but they weren't necessarily fun to use.)
Right now, there's a marketing void for people who like having a keyboard that doesn't cover the screen (especially having a vertical slider, which from personal experience with the Samsung i730, is just sweet), who perhaps liked Palm products, who perhaps are really not into letting Apple control everything, but who want an alternative that's fun and which should even be easy for beginners to program simple apps for.
The Pre fits the bill.
Where the Pre falls down, is with more intense apps like Slingplayer, games, and so forth. However, millions bought the iPhone before it had third party apps at all, so the Pre doesn't really suffer from that standpoint... yet.