I was wondering what percentages each platform makes up in your total sales. Not asking for sales data, just wondering how Linux fares in comparison to Mac OSX and Windows.
Is it one download with all versions or do you only download the versions you're gonna use?
If it's the latter it should be trackable (but only after the game is released).
One could also consider average visits from different OS's to the site (would give a general view of who seems to be the most interested group).