So we've got [barbarian] and [shaman] tags. There has been some degree of tag war about them. Are they useful?
My sense: nope, because they are inherently undefined. Does [shaman] mean 4e shaman, 3e shaman, AD&D shaman, or Greg Stafford? Clicking the shaman tag will not give me a well defined list of questions.
I think they are useful. They allow someone to follow a theme fairly easily by going to tag, clicking it and seeing just those question. The purpose of tagging is to provide non-hierarchical information that allows easy, and intuitive searches.
To be an effective tag is has to be intelligible (easily understood by anyone who looks at it) and, at the same time, limit itself. The class tags like shaman, warlock or barbarian do exactly that, and provide a good deal more intelligence about the questions that contain them than others I could think of (natural-1, what is that?).
As an aside I rather find the differences and similarities between games as enlightening as you find them annoying, which makes yet another good reason to keep the tags.
I think a system-agnostic [barbarian] tag promotes a useful search/browse workflow for players and game designers, if not necessarily people who are actually running games. "What kinds of things do barbarians do, irrespective of system?" is a question that can be answered most efficiently by examining questions tagged [barbarian]. If I'm new to R-pGs in general, and want more information about what barbarians are like, reading those questions will tell me how other players view barbarians. Similarly, if I'm writing my own R-pG, and want to see what mechanics other systems see as innate to "barbarian-hood," surveying a system-agnostic [barbarian] tag can be useful.
[dd4.0-barbarian] does not strike me as useful from a search/browse perspective. If we don't tag by class at all, I can search for "[dd4.0] barbarian." If we use system-agnostic class tags, I can search by "[dd4.0] [barbarian]."
Unfortunately, all that goes out the window when we talk about following. The following system is innately less functional than it could be. I don't feel like prefixed tags are a good answer to that lack of functionality, because it doesn't mesh well with my personal mental map of what tags do. Also, (danger: slippery-slope argument ahead) I don't see why, if [dd4.0-barbarian] is useful, we don't also want to tag the "when to use rage" question with [dd4.0-rules] and [dd4.0-combat].
I'm a fence sitter; I think the idea behind the tags is good but that the tags should be different.
When I was asking In what edition of DnD was the Barbarian's rage introduced? I originally was going to tag [dd-barbarian] but saw that [barbarian] already existed.
I think they should be present and we should prefix them.