Completely open source, more flexibility than the iPhone, multitasking, Google integration, etc.
Plus, I'm pretty sure the next iteration of Google Voice is going to have VOIP, considering their acquisition of Gizmo and the fact that Google is beginning to sell its own hardware.
The iPhone has a fantastic app store at its disposal, but it misses the mark on so many basic things that I want out of a device. I mean - jesus! - the iPhone just recently got proper MMS integration.