Anything that relies on physics with small objects should be avoided. To-scale billiards table = destined to fail.

Small scripted HUDs work well, either having the HUD be the entire game or having it interact with objects in the world.

Full-screen HUDs should be avoided though, because screen sizes vary and they can block other HUDs and things users need to click on.

makes sense.

Casual games that you can play with your friends, or that let you compare scores with friends, work well.

For marketing, a demo/free-to-play version is a very good idea. Especially a copy/transferable one that people can give to friends.