I think below are the application which can be really useful:

1. A decent lightweight browser.
2. Better mail client, which doesn't slow down N900 but work in background.
3. Some app to stop draining battery (put internet connection on sleep maybe) when connected to internet but only used by conversation plugin. (I have to always off internet to save bat which doesn't let me be online for much longer on N900)
4. If possible skydrive (onedrive) client cos it provide more storage and also it doesn't have server down issue like ubuntu one have (for me).
5. Application like easyanswer is there for harmattan (least required).
6. Any shortcut to close all open tasks maybe in power menu (least required).
7. Phone app should show call duration info so we don't need to go to extended call log for just that (least required).

Thanks