Here's a few things: - constantly changing things around. UI gets less intuitive for every release. - slower for every release - violates IMAP standard by re

Here's a few things: - constantly changing things around. UI gets less intuitive for every release. - slower for every release - violates IMAP standard by re-using IDs across tags/IMAP folders, risking actual data loss. Example: if you in a real email-client try to delete a email from a single folder, you will also delete this email from all other folders it has been "tagged" in. - similar IMAP issues with sent emails. Hard to track sent emails from email-client, unless you allow the email-client to explicitly save a copy to sent-emails folder. But then you suddenly have duplicate emails in the web-UI. - Makes an open standard (internet email) proprietary. - Hell to integrate with: See above. I'm sure I could go on, but really. If you still consider GMail best in class, the only possible explanation is that you haven't seen anything else.