A Hacker's Replacement for Gmail
I'm not sure why you can't do those things on FastMail.<p>(disclaimer: I work for FastMail)<p>Sure we have folders rather than tags, which means you can't add multiple of them to the same message.  Probably the biggest lack is that you can't manage IMAP flags via the web interface.  Otherwise, our search is now very powerful (since about March this year) and allows you to build filters that show messages from multiple folders in a single view.