The one final thing I've been wanting from Gmail has finally arrived - IMAP support! IMAP allows you to synchronise your email across different clients. So, for instance, if I'm reading a message from my Gmail account using Thunderbird and then delete it, it's also deleted when I look at my Gmail using the Web client.
This really has changed everything for me and I can now see myself using only Gmail as my primary email account. At the moment, it only seems to be available for the Google Apps version (i.e. Gmail for domains), but I assume it will be rolled out to all accounts eventually.
Here's a screenshot of the new Settings page:
Well done Google :)