Non DRM Mobipocket format is already supported on the Kindle - just drag and drop.
Typically ebook readers are larger than PMP's with larger screens. If you're going to read for a few hours, then the screen of a typical PMP becomes pretty eye-straining (due to size, contrast, and brightness).
If you read some of the Kindle discussion groups, you'll see people complaining about "no backlighting", and then a few days/weeks later they will be lauding the fact that it has no backlighting.
The Kindle has a built in cell receiver so that you can wirelessly browse, preview and purchase books using one-click.