Most likely nobody will be enraged until your game gets famous enough
. However, it's better not to lose players. Supporting multiple platforms is always difficult, even with Java, but let's face the truth: if you have, say, 100 players, then actually you must support 100 different platforms. So you need to keep your code as portable as you can, no matter which platform you target.
I think that we all should not stick to Microsoft too much. Yes, they make the best development tools and Windows XP is the best OS so far, but I'm afraid it will not be like this forever. Vista was distinguishably worse, Windows 7 was even worse than Vista and 8 is going to be a total disaster. That's why I'm struggling to make friends with Linux despite the fact that Linux folks don't understand the word "standard". And "GUI" apparently means "a nice desktop wallpaper" for them.