I am not sure which email client people think is so good on the N900, but Mail for Exchange is rubbish. It doesn't render full html emails. It can't accept appointments and it doesn't send appointments to other people properly. For such a major function (email) this is a major failure. I haven't used the mail client on Android, but could it be worse?

Mapping - there are several apps out there trying to be as good as Google Maps, but aren't there yet. Also there is a major problem with the stability of the GPS driver in the N900. Haven't seen it? Try running one of the mapping programs for five or six hours recording your tracking and you will see what I mean.

What's with the OS getting so slow after you have used it for a couple of weeks, such that you need to format the N900 to get your speed back? May be this caused by MfE, but it is a real killer that OC won't fix.

Stuttering in audio playback.

Lack of specialist/localised apps as stated above.

Voice search

So much hacking is required to do what is basic on many other devices (ie setting voip codecs etc)

Hardware Side
Crap front facing camera. Sure we have Skype video, but the camera is really dark/insensitive.

Small screen - 4 or 4.5 inch

Refection around camera lens.

No compass