You don't need to switch between accounts anymore. Mailplane 3.0 is able to keep multiple accounts open at the same time. And on top of that it supports Google Calendar, Google Docs and Google Reader..
Attaching files is now much easier. Mailplane 3.0 keeps them locally until you send the email. Thus, it's easy to preview, edit and resize them.
Gmail allows you to put attachments inline. Mailplane 3.0 supports that feature.
2-Step Verification does increase the sign-in security of your Google Account. Mailplane 3.0 supports that natively. See http://gmailblog.blogspot.com/2011/02/advanced-sign-in-security-for...
New Downloads Window
Mailplane 3.0 has a Safari-like Downloads Window.