I saw that a bunch of questions about apps for Android have been downvoted. Is there any clear idea on what is the scope of this site platformwise?
Possible scopes would be:
Recommendations for Windows programs only?
Recommendations for programs on all operating systems running on a PC (or Mac?)?
Recommendations also for mobile devices "the way they are meant to be used"?
Recommendations also for mobile devices software, that only runs on a jailbroken or otherwise modified operating system.
I personally think the first two points should be on topic in any case and I see no reason why the third should be excluded. In the fourth case I think it would be okay, but the asker should specify in the question when those answers are acceptable.
I see no reason why not, similarly to language tags on SO, game title tags on Arquade, or franchise tags on SciFi. Just require a platform tag with each platform-specific question.
There may be questions with the platform-agnostic tag, where you're willing to purchase/build any system/setup which provides platform for software with desired functionality, and cross-platform for interconnection between two or more platforms, but I believe simply most of questions would need to be tagged windows-server, android, debian, and so on.