My intention was more "why haven't desktop apps copied the best parts of GMail and made it irrelevant?" But here are the features I'd like to see in a desktop client:

Conversation view (one line per conversation in folder view, all emails in conversation shown when viewing the conversation, and the emails are sorted by most recent message, etc) Fast (fast search, fast email viewing, no pauses while waiting for emails to download, etc) Simple interface (single screen which changes views)

The thing I'd like most that gmail doesn't have (even with offline gmail) is to have my mail stored locally and be able to use it while I'm not connected.