##Wherein I address some of the concerns raised on other answers

(This started out as responses to Kit Menke and Anders Rask, but... it got long)

First off: this is our fault, at least in part... One of the big goals of this beta period is to let the site develop without the baggage of SharePoint Overflow to hold it back - and yet we missed the opportunity to warn you away from this tagging strategy earlier. I sincerely apologize.

Second, this is... kinda urgent. Versioned tags have caused headaches on other sites, so it's important to tackle this issue before the volume of questions is too large to deal with. We're set to dump a whole lot of old questions from the SharePoint Overflow site, and they're going to need some cleanup - that includes tags. The 2010 tag is gone, and the 2007 tag will have to follow. It would probably be a good idea to consider the development tag as well...

I believe many users won't be able to tell when their question is version specific or not.

This is probably the best argument against version-specific tags. If you're used to answering SharePoint 2007 questions and browse the site looking for that tag, you're going to miss any question that isn't specific to that version. By assuming most questions are version-specific, you resign yourself to a future where all questions are version-specific! No matter how good your intentions might be regarding cross-version duplicates, you're setting yourself up for failure by creating a site where users are encouraged to ignore a giant chunk of the questions.

However, instead of discouraging the version tag we should encourage adding more than one tag. What if we made the minimum number of required tags TWO instead of one?

This just punts the problem on to the next lousy tag... Which makes this a good time to bring up development, now the second-most popular tag on the site. And it tells you even less about the question than the version tag does. And yet, there are questions on the site tagged with nothing but [sharepoint-2007] [development] (and up until recently, the same situation existed for [sharepoint-2010]).

###A philosophy of tagging

Tags are intended to categorize questions, allowing experts in specific, well-defined categories to follow questions in those categories and users to find them. A well-tagged set of questions will expose the focus of the site, creating a dynamic, flexible, non-hierarchical organization system.

Stack Exchange sites require users to enter at least one tag: questions should start out in some category, and more can then be added to refine or link in related topics. It's super-important that this step doesn't inadvertently encourage users to miscategorize their question by presenting them with ambiguous or misleading tags.

It's worth noting here that the system already blocks certain redundant tags. Obviously, tagging a question [sharepoint] on a question named SharePoint isn't very useful... But that's also going to be the first tag that occurs to most users, effectively guaranteeing that [sharepoint-version] will see widespread use. It's actually sort of a clever tactic to get them to pick from a list of pre-defined options...

So instead of adding noise in the Q ("i am using SP2010") and delaying answers with ping-pong comments with "what version are you on" we decided to keep versions.

That's understandable, and as I said, somewhat clever. But... You're sacrificing one entire dimension of a question's information for the sake of reducing omissions in another dimension. There are all sorts of details you'll want regarding problems users face; among them, which version of the software they're using has to be one of the easiest and most obvious.

##Going forward, cleaning house

As I mentioned at the start of this, we're about to re-introduce the old SharePoint Overflow questions. Before that happens, you'll want to have your house in order: the new questions should be well-tagged, top tags should be clear, specific, and easy to choose from, with any necessary synonyms or black-listed tags configured.

To that end, I strongly suggest you consider eliminating the [sharepoint-version] tags entirely. Don't make them optional - they'll creep back into every question where a user hits that tag box and starts typing "shar...". As Jeff notes, this elimination can be done easily upon request.

Following that, take a good hard look at development - can it be eliminated, renamed, or at least documented (I know what software development is, but what does the tag mean here?) Keep in mind that meta tags are also considered harmful - if the tag doesn't describe the content of the question in a way that can stand alone as that one mandatory tag, it should be eliminated.

And finally, attack every question without a good, solid tags, and give them something that actually categorizes them, based on the content itself.