(i)The screen scratches more easily than capacitive screens. I have two very large scratches on my screen despite making sure I put absolutely nothing in the same pocket as the N900. Be sure you use a screen protector because no matter how careful you are the screen may get scratched.
(ii) There is a serious manufacturing defect with the micro-usb socket of many of the N900s.
(iii) Only three row keyboard layout so you need to use alternate for the numbers keys.
Battery If you do not have IM accounts turned on then the battery should easily last you through the day. However, with some IM accounts (not sure which are the culprits but yahoo and msn seem to be amongst) you should generally expect around 6 hours use on average. Not really an issue specific to the N900 though - Engagdet's review of the HTC Desire says they had 6 hours with similar use.
(i) Some devices randomly restart.
(ii) The transition between the four desktops is far from smooth.
(vi) No proper functioning software to sync with your PC the way itunes syncs perfectly with iphone. You can use drag and drop but that is only helpful for the first sync of all your music and when you add new songs to your PC and are sure exactly what the news songs are and can just drag and drop them. There is Ovi Music but it does not work with the N900. Third party PC software programmes can sync but many of them just sync to the main documents folder or the main music folder rather than in an organised manner (via artist, album artist, etc) and so create a huge mess and will kill your use of file manager. You may get something to work properly but will be quite an effort compared to using itunes and iphone.
(ix) Cannot dial numbers starting with * or # without the use of a third party widget so without the third party widget you are unable to top up your credit or check your balance in certain countries.