Well, so here is a small listing of features I’d need before I switch anywhere from Thunderbird:
Support for HTTP/HTTPS and Socks5 proxies (thanks to ISP restrictions)
Better conversation support compared to Thunderbird esp. for mailing lists (actually something like on gmail would be nice)
Full-text indexing or at least tagging
Plain-old POP3 support as long as gmail doesn’t offer SMTP ;)
An open storage system
I think this should be it. For me esp. Socks5 and general proxy support is very important considering that this was also the main reason why I never could really try out Apple’s Mail.app. You might notice, that I don’t list a spam filter. I started deleting any mail I’m not interested in anyway, so spam filtering is something that I could entrust the server with ;) Sure, spam filtering is nice, but there are already enough frameworks out there. Perhaps some interface to some of them would be nice.