Coming from a G1 to N900, here's what I dislike on the G1 (not necessarily Android):

- battery life is too short
- capacitive touch screen responsiveness varies with the weather
- takes forever to boot (OK, you don't reboot a phone often unless it runs WinMo)
- sluggish UI
- poor multitasking experience due to apps getting restarted after switching to them due to not enough memory
- approx. 70 MB for installing apps, this space gets filled in no time