I am not a fan of Gmail, its got too many things wrong with it. Most of which I've mentioned in the "suggest a feature" page, and all of which have been ignored (at least to all appearances).
- No real threading
- The spam filter has too many false positives
- The spam filter has too many false negatives
- The user filters can't filter on arbitrary mail headers. This means that mailing lists have to be sorted on subject (so you can't put responses to list messages sent directly to you elsewhere) or To address (oh dear, BCC)
- When you tell Gmail that a message is not spam, it puts it back in the inbox - even if you have a filter which tells it to bypass the inbox.
- When looking at the spam folder there is a "This is not spam" button. If you click on a message to see if it is spam or not, that button moves to the right and is replaced by "Delete forever!". I've lost mail I wanted to keep thanks to that.
- Sometimes it gets confused and shows you all the messages in one folder as if you were looking in another folder. So you look in A, it thinks you are looking in B, and shows you all the messages in A with explicit "Also in A" labels, and the colour scheme of B (OK, this one hasn't been ignored. They asked me for a screenshot. I haven't heard anything since I sent it though).