This is mostly an extension of badp's answer, in so much that it highlights our lack of real tagging capability. However, it's a bit too long for comments. I'd also like to point some attention to previous posts on tagging practices, discussing some parts of this issue.
In other subject matter, there are lots of terms that are both mostly universal to the topic, and which actually make some substance to ask questions about. You can work with strings or sorting in Stack Overflow, collision detection or audio in Game Development, meat or frying on Seasoned Advice, etc. You can often have one tag that represents the super subject (language in Stack Overflow or Game Development, food type in Seasoned Advice), then consequent tags work to narrow down within that.
The luxury is not nearly as present in the field of Gaming. Once you get past the super subject (the game), there aren't many topical items that are both universal to Gaming as a whole, and which make substance for questions.
There are lots of things which can be found as universal in meaning to Gaming. Double-jump mechanics, hitboxes, triangle jumping, hitstun, critical hits, post-damage invulnerability, falling damage... there's a lot that gamers are familiar with that are carried across all games. They're also things that don't make for very interesting or sought-after questions. There's just not much interesting to ask about when it comes to double-jumping or post-damage invulnerability. Except for exotic terms like Kiting, which only exist to this site in order to be asked what they are. This affects a large fraction of universal gaming terminology that would make effective tags in the Stack Exchange sense.
Then there are a lot of concepts which do make for interesting questions. How bunkers work, doing a barrel roll, the function of a scout... you can find some interesting things to ask about. The problem is the lack of universal application. The definitions of all of the bolded terms vary so much between games that the tag, as far as how tags in Stack Exchange work, loses all meaning. Tags don't mean "everything that is called this", but reference a super concept. Other sites that deal with gaming material usually handle this by adding qualifiers on the tag (so, scout becomes
scout_(team_fortress_2), for example). We don't have the tag length to reasonably permit that practice, which makes it pretty much folly to use many tags like that. And, consequently, the large fraction of subjects which our questions do deal with fall under this category.
We aren't completely absent of usable sub tags. Weapons, random drops, minimaps, achievements, invincibility frames... all of these terms do carry both a universal meaning to gamers, as well as make for interesting questions. They're just... not that prevalent in questions, it seems. As such, even though we try to follow the normal practices of Stack Exchange for tagging, it's not readily visible.
Ultimately, the basic tag rules that I most often use, and which I encourage, are the following. I do admit to having broken the platform on a few occasions when it came to a platform exclusive game.
- Tag the game, if you're asking about a specific game.
- Tag the aspects of the game which you are asking about, if possible (boss-fights, multiplayer, plot, etc.).
- Tag the platform if your question is specifically about one platform, or about one platform's exclusive features for a game.
- Tag the genre if your question is specifically about that genre.
- Try to avoid tags that are ambiguous (many of us work on disambiguating various tags).