Access to more apps. There is a large repository of Debian ARM apps. While these are not written for the N900's hardware specifically, many of them will work OK. Some standouts (if I understand correctly -- my experience is with Easy Debian for the N810):
* OpenOffice.org, which has similar functionality to MS Office
* The GIMP, a sophisticated image manipulation tool
* Ability to run Java apps via Iced Tea

However, you need to do your homework as it is a hack. There are lots of long forum threads that will be your friend.