Generally speaking, should tags be in the form of
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which way will users search for this term on the broader Internet?
The most important factor by far. For example, I'm about to rename a bunch of tags on Server Fault which are in the form
I do not believe anyone types "windowsserver2008" into Google (or Bing, or whatever). I believe they type
Windows Server 2008
Which means the appropriate tag is
.. because dashes are treated as word breaks in every known search engine (and regular expressions, since forever). This is critical to get right because it means people will be able to find what they're looking for.
which form is more popular?
In the case where the search argument cannot be made -- for abstract terms, or technical terms that tend to be a "lump" without word breaks -- I tend to argue "survival of the fittest". Whichever tag has more questions associated, whichever tag is used by more people, should win.
Users will inevitably try to create all variations of a tag, mostly unintentionally. That's was the basis of my original suggestion (Do 23,000+ Tags Need Tag Database Editors?).
Definitely come up with a simple statement of the preferred way to name all tags and let the "editors" nudge the tags into that definition. Edge cases can be handled individually. Ideally, this standard would fit in a ~10-word statement that you can place under the tag entry box. Something like...
Enter Message Tags: [___________________________________] Use hyphens for multiple words (i.e foo-bar). Avoid abbreviations.
My personal opinion:
If the terms "foo" and "bar" are distinct, I think they should tagged as "foo-bar" like "visual-studio" or "vampire-slaying".
If the terms are together, they should be tagged together like "coldfusion".
Basically just replace space with a hyphen in tags. The tag text box isn't really specific about how to combine words:
Combine multiple words into single-words, space to separate up to 5 tags (python c# ruby)
If you want people to be consistent you need at least an example of a multi-word tag in the parenthesized list of examples.
I would imagine that you would want to stick with whatever was the most popular and continue to push that as the convention for that particular set of tags. In some cases a separation of the words is ideal (such as feature-request) but in others it is not as good (visualstudio seems to be a much more favorable tag than visual-studio in SO as well as all of it's "subtags")