I see a lot of people use the standard task application in their e-mail client. This forces you to keep e-mail open all day long, looking over what to do next multiple times a day. Side effect is that you're tricked into directly reading new e-mail as it arrives; a pure distraction nightmare, if you ask me. Therefore I always tell people to use a separate task system, be it paper or digital. It enables me to move beyond e-mail and handle all incoming information in the same way.
Of course you can turn off "you've got mail" messages, but even then you probably go and check e-mail dozens of times a day...
I'm curious if you agree: task applications in e-mail clients do more harm than good when you want to be really productive. Therefore they are better left unused.