The Amazon Appstore looks really nice; the large product description section (for some), the review system, the links to other apps of interest, all far more nicely laid out. I don't know if there's a notable difference in inventory compared to the Android Market, but when it comes to presentation and shopping experience, Amazon is showing why it's Amazon. I think this is a good thing for app developers, too.
Look at the difference between the product pages:
Angry Birds: Android Market
Angry Birds: Amazon Appstore
nicely done, Amazonians.