Number one which is a massive problem is battery life. I bought my phone a week or two ago direct from nokia, and i wouldn' class myself as any more than a moderate user. Still, on an average day I can listen to a bit of music on the way into work, and get some usage out of it at work, and thats it.
The worst thing is during my initial excitement I persuaded a colleague to get an N900 and his phone performs worst than mine on 100% factory settings. 5his brings me on to my second issue which is the attitude on this site of 'its not a phone its a computer.' I keep seeing this on any thread describing a shortfall with the N900 and its driving me mad because its a load of crap.