So, I have a lot of problems with Google docs on a philosophical level, but here's a practical one:
They do user-agent sniffing.
I use Firefox Nightly (cutting edge release), and Google Docs works fine... unless I block my user agent string (which I normally do for privacy reasons). Then it renders in compatibility mode and I can't edit anything. This is dumb.
Google, stop being dumb. I'm currently announcing to Google Docs that I'm running Firefox 4 on Windows NT, and that seems to make it happy.