It seems no-one has remembered to tell you the biggest single difference between projects that belong to Game Development and those that belong to Game Demos.
To Game Demos you must upload the whole source of the game, the ged-file and the data folder with it's contents. To Game Development you can decide how much you want to share of your game. That's why many people share their work-in-progress / test versions of games in Game Development and the final versions in Game Demos. Most people don't want to show unfinished, probably somewhat messy code to people before it's finished and cleaned.
But some people who don't want to share their code even decide to publish their finished games in Game Development as only executables.
But besides the limit of having to have the whole project uploaded when postin to Game Demos, there's really not that much rules or limitations to it. If you look at demos other people have posted, there's a lot of demos of neat ideas people have had, and examples of how to accomplish things.
The Games-page requires you to have a topic where to link, so having your game on the games-page goes well along with having a topic for the game in Game Demos. You want to be able to say something about the game to people who are about to try it, right?
(Excuse any misspellings, I wrote this on my phone)