Hm, maybe you can't define the term "retro" but you KNOW what it is "retro" or not when you SEE it. For instance, your Popeye game is definetely NOT retro for me. I think the look and presentation as well as usage of "old" techniques is very important to be considered "retro". E.g. the new Megaman game is brand new but also retro as hell, they even simulated flickering and limited sprites like on the NES. Consider making a Popeye game in 2D with no interpolated sprites or no sprite rotation and sprite animation and that stuff with old skool 8bit-music :-) that would be a charme!

I think JCL/George should -not- define what "retro" is, because it is a style, a feeling.. the users should decide.

Just my thoughts.. what is important are some clear instructions regarding legal issues. The rest should be relatively loose. Haha, a screen resolution limitation would be neat, like 256x224 for the SNES ;-)