When I originally got my N900, it could offer a combination of things that few other phones could at that time.

Since then, smartphones have fully entered the mainstream, lots of stuff has developed massively (Android for example) and for me, many phones can now offer the things I want.

So, it's likely to come down to the one feature that a) is of significant interest to me and b) the quality of which varies substantially from phone to phone. And that is the camera.

Although, I do also like having a hardware keyboard and resistive/stylus optimised screen so these things could also come into play.