I don't know whether the categorization through tags as currently used helps anybody -- there seem to be an awful lot of tags and even reading through the list takes ages.
As far as I can tell, we have tags of the following sorts:
Tags for the old menu categories or sub-categories such as "filter", "blur", "blend", "color" and so on
Various functional tags such as "grain removal", "generate", "stereoscopic", "resize", etc. This is what I personally would have expected as tags and it is quite a large category unto itself.
Version numbers, such as 2.4 or "gimp 2.x". I am not sure whether these are meant to apply to the plugin or the gimp version. In the latter case, they are a duplicate of whats already available in another category. If they are meant for the plugin, that is quite confusion.
Type tags such as "script", "script-fu", "plugin", etc. Again, these are a duplicate of something that is explicitly given before. I wonder why people add these?
Language tags such as "python" and "perl". More duplication, as this is already implied by the "scripting engine" category.
Wrongly separated tags such as "2.6 tiles generate wallpaper desktop" or "dvdauthor spumux dvd menu Python-fu titles" (all as one tag). These seem to be the result of user error, caused by the inconsistent usage of spaces vs. commas as separators on the various tag-supporting sites.
Tags that duplicate the plug-ins name or effect, e.g "Che Guevara", "Lightsaber" or
Pure numbers, e.g. "4444" or "5551". These appear to be used by just one plugin, the RAWTex plugin, which uses them to mark RGBA formats.
Well, I may have missed a few types of tags but even so, a picture emerges.
A thing I truly wonder about is this: Why do people add tags for things that have their own categories, such as the license, language and gimp version? I have no explanation for that. For the other things, what I would have liked to do for some of them is to add explicit categories which users can select from. However, if users would still add the same things as tags, that doesn't make sense.