Petrovich and that whole sharing account tie-in
Pidgin Integration(for Facebook Chat, Skype, Google Talk, and AIM)
Joikuspot would be cool
Hermes(or an equivalent for pulling all your Facebook contacts into your phonebook with pictures of that contact)
A File Explorer
PDF Reader
And is the Conversations "paradigm" still there? Does it look at all your IMs and texts as the same thing?
No copy-paste, except for a few text input fields. This is very annoying especially in the browser, where text input is fundamentally broken.
No cursor keys on the onscreen keyboard. The replacement (a lense) doesn't work in all applications... again, the browser is the most important application in which you cannot control the cursor position at all.
The display is the worst I've ever seen on a mobile phone, much worse than the wonderful display you enjoy on the N900. Color bleeding, shadows and stripes, purple tint, whatever... there's a whole thread about it.
Also lacking from the browser is the 'mouse over' mode you have in the N900, flash support of course, any way to edit bookmarks (actually, there's no such thing as bookmarks)...