I believe that tags on SO should be relatively rigid.
In general, when tagging media like images, audio, or video that is not otherwise easily indexed, you want a very loose tagging structure that encourages adding as many and as varied tags as possible. This is because you lean heavily on the tags to index your content for search. It's why flickr and youtube let you tag things any way you want.
But when talking about text, which is already easily indexed, the tags themselves don't matter for indexing; you index the text directly. The tags don't matter for summarizing; that's the job of the title. Instead, tags are used strictly for categorization. Categorization is a different thing entirely, and it only works if everyone is sorting content into the same set of categories. You need a rigid set of categories for this happen. It's why gmail labels (which are really tags) are harder to just create willy-nilly.
StackOverflow content is definitely text-based, and that means we should be thinking of rigid tag categories rather than a loose tag cloud. I've advocated a few changes to the tagging system to help improve things:
- done Minimum rep threshold required to create new tags (implemented at 100, I'd like to see it bumped to 250 so that an eager new user has to spend at least more than one day with the site before they can create new tags).
- done A better explanation of why the question was rejected would be nice, too.
- Add a warning whenever using a tag that appears on fewer than 10 other questions.
- Restrict certain junk words like 'and', 'the', 'a', 'server'(on stackoverflow), 'visual', etc from use as tags entirely. Possibly even do this silently where possible- the tag just doesn't show after you post.
- Some synonyms are already tracked and corrected automatically every hour. This should be done as questions are posted and edited instead. This way, users are more likely to notice the tag changed (often the entire question is resolved before the correction, so the user never knows about it) and we reduce time spent by community members correcting these tags that could be more profitably spent elsewhere. This is also important because by the time a correction is applied the question has fallen off the main page, and users that might have been interested in the question based on the corrected tag will never see it.
- done A way for users to nominate tags for merging/elimination/add to synonyms list built into the user interface of the site (not just posting a question here) that allows high-rep users to comment and vote on the nominations via the 10K tools.
- Re-name 'tags' to 'labels' ala GMail, to encourage users to think of them in terms of categorization.