Watching a number of colleagues and friends deal with email recently, I found out that a lot of people still use their inbox as a kind of to-do list. Whenever an email comes in, they take it as yet another task.
There are a couple of problems with this. Most have been described at length in several posts, such as the Inbox Zero articles by Merlin Mann at 43Folders. You may want to read some of his excellent articles, which align with most of the ideas in David Allen's GTD methodology.
What really gets to me in using your inbox as a to do list is that you actually allow others to determine what is the most important thing you will be doing. That comes down to giving control over your activities to other people.
In essence, using your inbox as a to do list comes down to saying yes to everything that anyone cares to send to you. Even if you just consider it, you are still considering it at not the time of your choosing, but the time they impose on you.
And that, to me, is just too much control in other people's hands.