As an ex N800 owner who's now upgraded to an N810, I can say the keyboard and improved screen make a big difference.

For isntance, chatting on Skype or Google Talk or even just typing in web addresses is a whole lot more easier to do. The on-screen keyboard on the N800 is awful. With my old N800 it would mis-read taps or double-tap letters for me.

The GPS is quite handy too, especially when combined with Maemo-mapper.

Plus the N810 looks nicer

The keyboard is the main thing though, it really is worth it.