N900 is a phone for tweakers really. You can adjust the phone OS to your likings, but there are ofcourse a great deal of things you'd expect from the N900, but actually aren't there.
RolePlayGame already mentioned a few.
If you like to hack settings or develop software for the N900 than that's the phone to go for. If you like WOW-factor, 1-hand control or ease of use then I'd suggest you take a look at other phones.