I think a requirement of supporting multiple platforms would benefit a lot of people here, but, I get the idea that many constestants have never used anything besides Windows. Since they would need valuable time to get experienced with another platform, this would prevent many developers from entering the competition. IMHO a multi-platform requirement is therefore a bad idea.

However, the current situation is not really fair either, since being multi-platform costs you extra time to develop and test for the other platforms. Therefore it would be a good idea to reward people.

It could be done with a formula, for example:
* 50 points or somthing like that are rewarded for multi-platform development.
* The game that supports the least platforms gets zero points.
* The game that supports the most platforms gets all 50 points.
* The score for games that are in between gets interpolated, i.e. score := 50* (platform_count-minimum)/(maximum-minimum)

For example:
* Game A supports Win32.
* Game B supports Dos and Win32
* Game C supports Linux, FreeBSD and Win32.

Game A supports 1 platform, B supports 2, C supports 3. The 0 points go to game A. Game C gets the 50 points. We interpolate for game B: 50*(2-1)/(3-1) = 25 points.