Palm has even written a guideline of what they want apps to have. They should be useful and engaging with an "appealing design" and a UI that meshes with the Palm UI. They must be written for webOS and cannot be downloaded through the browser. Apps should take advantage of the device's features like multi-tasking, accelerometer, etc. Finally, the app must be responsive and should not lag or be sluggish. Instructions on how to submit an app as well as ideas on testing your submission are written at the source link.
source: Palm Developers Network Blog via mobileburn