I didn't realize this until I tried posting on some Stack Exchange 1.0 sites recently, but apparently Stack Overflow doesn't allow new users to create tags. To be honest, it's kind of annoying. But I suppose annoyance isn't a big deal if there's a valid reason for this. So, what is the reason for this limitation?
The reason this was added to StackOverflow is that if you have to create a new tag, you're probably tagging your question wrong. Remember, tags are not for summarizing or indexing your question. They are for sorting your question into existing, well-defined categories so that users who watch those categories can find your question and give you an answer. Tag your question wrong, and you're seriously hurting your ability to get a good answer.
I'm not sure this reasoning holds up as well for StackExchange sites, at least not yet. Most of those are much smaller, more focused, and tend to have more questions where anyone can answer with an opinion, such that every user can follow nearly every question. Over time some of those sites may grow and the tags may become more important.
If I remember right, before that limitation was in place you'd get 80 different ways of expressing the same tag - mostly from people who couldn't bother to get the hint (pun intended) and/or just wanted to create the new tag for reasons only known by them.
This at least encourages the new user to look at what tag names the community is consolidating on.
For a site that has an existing user base, most tags have been created, and we don't need redundant tags created over and over by brand new users. If it really needs a new tag, a user with higher rep can retag it appropriately. The amount of rep required to create tags is fairly low relative to re-tagging and editing others' posts though.