A single click to open a message works great for me. (Vista/IE7)
Also, I like the ability to select the third column and have the Compose screen open. If it is a draft I have been working on I can go back to it with a single click. If it is an email I wnat to respond to, I can go to a reply screen with a single click. Nice feature that requires a single click to get to what I want to do!
However, once I am in the compose screen there needs to be options that allow me to automatically insert the current email text if I am replying and to select the identity I want to use. It should default to the 'to' address of the current email.
A great start!!
David