Amazon is waiving the $99 program fees for the first year which is a very welcome move . 

Why Amazon Appstore for USERS ?

1) Currently Android Market does not allow you to down apps onto your PC. But with Amazon you can download apps both onto your PC and from your mobile devices.

2) App recommendations based on previous purchases, browse, search refinement etc. makes it possible to find apps.

3) All Android devices with OS 1.6 and higher are supported.

4) Both paid and free apps are available in the Amazon appstore.

5) Amazon Appstore Distribution Agreement is non-exclusive. Hence all apps in the Android Market can also be found in the Amazon Appstore.

What is the criteria for approving apps ?

Nothing has been said in this regard but my guess is that they will stay away from pornographic and controversial apps.

Here are the step by step instructions on how to submit  your app to Amazon

Submitting your android app to Amazon Appstore