I can't really think of 5 things, but:

1) Flash 10.1 compatibility at least (with or without gpu accel depending on how much difference it makes.

erm.......

2)general improvements in responsiveness (Although I've yet to upgrade to 1.3 so not sure how much has improved..)

For example I just missed a phone call because my n900 was busy, so the answer button took too long to be redrawn. As limited as the iPhone is, it always seems responsive no matter what is going on..


Ooh!

3) USB OTG with support for mice, keyboards and VGA out. Would be great to be able to plug into a docking station with those 3 connectors..