for me, the 3 major weaknesses (leaving aside the o/s and focusing solely on day to day usage) are:

1) video out - the HDMI should be way better than the n900 but in practice just isn't - for me anyway

2) skype performance: n900 skype was bullet-proof, even on 2G I could always connect 1st time and hardly ever experienced call quality issues. on the note I find that skype audio calls (mainly to telephones rather than other skype users) are useless in "real-world" conditions. wifi is fine but I need skype to work on cellular network and it always lets me down. even stationarh with a strong signal and > 1MB/s download the call often doesnt connect and the audio is normally unintelligible if it does.

3) audio (both through speakers and headphones) is inferior to n900. out of the box, the note audio is tinny at best and it needs allthe help it can get from EQ to make it liveable-with as your main mp3 player

I'd also echo all of Ernesto's points above but the note is still the best thing out there for fully portable surfing/eBook/sat nav/movie player/ mp3/remote control/sketch & notetaking - there's simply nothing else with the screen size + quality plus s-pen plus fast-lock GPS in a pocketable form factor. I would have stayed with my n900 until it died except that the screen size of the note makes surfing, remote control and ebook reading soooo much easier on the eye. I never read eBooks on n900 and only did really essential remote access tasks - but the note wins for me because I no longer need carry real books i.e. one step nearer total convergance.