Ferris Research Weblog says there are two: “push technology and a useable keyboard.
You simply need to look at the success of the RIM BlackBerry platform, the current king of the hill when it comes to mobile email.
The BlackBerrys push architecture makes synchronization a thing of the past. If your BlackBerry is powered on and you have network coverage, your device is in sync. This is a very good thing because synchronization is a concept that non-technical users have never really wanted to deal with, and frankly never will.
Once you have push, some type of keyboard is absolutely essential. SMS from a numeric keypad maybe fine for the kids but it’s not for most professionals. Unless youre comfortable conversing with your colleagues exclusively in the Internet shorthand of leet, a QWERTY keyboard is a must to compose responses to the emails that have been pushed to you.