TechCrunch says the store's simplicity is actually a selling point -- iTunes has ballooned up to do way more than just sell music, and in Doubletwist it's much more straightforward. Here's the other bonus for Mac users: while we originally had to wait for an app release, the music store is currently only available on the Mac version of the software. A PC version is coming soon.
It's doubtful Doubletwist will replace iTunes for many people, given all of the various things iTunes does for a Mac user. But then again, if you are using the Amazon MP3 store with any frequency, Doubletwist is probably worth a look: it's a free download on the website.