According to ExtremeTech reviewer Jim Lynch, the clean and easy interface, and built-in Google Search.
In his extensive preview of the upcoming free Google Email client, Lynch singles out "conversations" as one of the most innovative aspects of the new service.
Conversations are a dynamic and unique way of presenting a set of messages all covering the same subject. They borrow from threaded message boards, and can really help users navigate through long chains of similar stuff.
In addition, Lynch found the keyboard short-cuts useful, and the email preview mode interesting. Tackling the controversial context-sensitive ad system, Lynch said "We have absolutely no problem with how Google is handling the placement of these ads."