Off to a great start so far. Here are my suggestions:
-- Undo needs to allow you to undelete the last deleted message and allow you to undo a send for X seconds.
-- GMail address book browser. I need to be able to easily browse and edit my contacts as well as add a new contact from the currently active message.
-- Labels (yes, I know it's in the roadmap already). Especially and interface to rapidly add and remove labels like on Quora.
-- Plain-text emails
-- Mouseover/hover states for the menu buttons.
-- Double-clicking for account preferences is not intuitive enough.
-- Greater visual separation between read and unread messages. A little blue dot isn't sufficient.
-- Ability to set how often is checks for new messages. Setting for 30 minutes helps eliminate distractions.
-- It can be kept minimalist by only permitting certain actions via keyboard shortcuts. For example, there is no reason that bold and italics can't be made available via just CMD-B and CMD-I.
Question: When I delete does it delete just that message or the whole thread like in web-based GMail? If it deletes the whole thread, how can I delete single messages out of a thread instead of the whole thread?
If this progresses as I think it will, it looks like it will be worth buying. How much do you plan on charging for it? $19.99 is the sweet spot for me. You might want to use the Van Westendorp method to determine how to price it.