What should and shouldn't go into official themes?

[ninetydegrees]

I've already asked this question before but never got a clear answer so I'll ask again: it was my understanding that themes were just about colors, fonts and images and, until very recently, there was nothing else in themes.s2 files. However, now we have some themes which change text, presentation options, module positions and maybe even the modules which are displayed and those which are hidden. I simply wanna know if all options are considered themeable because I need guidelines when patching. Besides, I think it would be nice for designers to know that too.