I'm still using mine as my only phone, with kernel-power, overclocked and many speed tweaks to the OS. There's not much functionality a new phone would have that you can't get by installing apps from the repos...I'll just point out the troublesome parts.

Browsing isn't a surefire thing anymore. YouTube doesn't work well on the browser even with some of the later leaked Flash releases. If MicroB doesn't work with a site I use Macuco, but that isn't a sure thing either - it seems to have trouble with newer SSL certificates/protocols. Sometimes I have to fall back to Lynx on Debian chroot.

There's no Whatsapp anymore (RIP Yappari) and it's too popular to ignore, unfortunately, so that's an annoyance.

Bluetooth was always a bit limited and troublesome on the N900 (it works more like a desktop PC and doesn't work well with a lot of "phone gear," especially headsets) and it hasn't been updated to work with many newer devices.

Other than those, no problems really.