The basics:
GSM calls, Skype calls
Email, web
MP3, video
Taking photos (the camera was one of the best at the moment, but now it's surpassed by the competence)
GPS navigation

The advanced:
Logging by ssh to my personal server
Some Qt programming, nothing professional, it's just to myself
Annoy some my friends that owns iPhone 4 by telling them why N900 it's superior :-p

What I like specially:
The full keyboard!
The Maemo OS and it's openness
The web browser
Easy Debian
H-E-N (for usb host, tested with various flash drives)
Community SSU
I can run Meego and Android on it besides Maemo
Great storage capacity (32Gb + microSD)

What I don't like:
The abandonment by Nokia
Battery capacity (but it isn't worse than with other smartphones)
It could be thinner and with better screen contrast