The size for a start. The N900 is one hell of a brick. Sure, compared to what we call over here the Nokia Fridges (the previous IT and the Communicator range), the N900's dimensions can appear to be diminutive. But compared to the average phone that "normal" people use, it's a beast of a device.

The OS, although impressively stable for what is still an experimental device, is still quite immature. The lack of a proper portrait mode is sure killer for a mainstream smartphone. Although I'm a geek and am used to landscape mode, having owned a Nokia E90 for close to 2 years, I have to say that not being to quickly read an SMS or place a phone call in portrait mode (the unlock screen only works in landscape orientation) is really annoying.

Some basic functionalities still need a lot of work. The email client for example is close to unusable with a bog-standard Gmail account (via IMAP4) because of performance issues (which will hopefully be addressed in the next firmware release). It also lacks really basic features such as a search functionality.

The GPS / mapping software bundled in the N900, which is something that most people have come to expect in a high-end smartphone, is extremely poor. Ovi Maps on Maemo is so incredibly slow, sluggish and crippled that it's, in my humble opinion, close to useless. It's certainly the functionality I miss the most from my previous smarpthones (Nokia E90, 5800 and iPhone 3GS).

Maemo sure has a lot of potential. It also already has some really cool functionalities that are anything but geeky and would appeal to the mass-market. But it still has a bit too many rough edges in my opinions. It would probably be a mistake form Nokia to market this as a mainstream device. I'm guessing that the last thing Nokia would want is to see is Maemo labelled as yet another failure that's buggy and rushed to market.

I'm personally using the N900 as my main smartphone even though I've also got an iPhone 3GS but that's because I'm a geek, I really like the open approach that Nokia is taking with Maemo, I quite like the UI concepts in Maemo 5 and I just don't get on with the iPhone for some reason. I'm on the other side very hesitant in recommending the N900 to other people as I really feel that it's a tad too geeky and too rough for now. Should hopefully only take a few months before Maemo graduates though.