Currently, you only need 300 reputation to create new tags. If you couple that with the 500 reputation required to retag questions at your will, you can easily end up with lots of tags that should never exist in the first place.
Once the new tags get created, newer users start using it all over the place and we would end up with having to awkwardly deal with them through mega retag threads that aren't pleasant to handle for the community or the moderators. Having to cleanup after poor tags essentially distracts the community from activities that are generally more productive, like answering questions.
(Did I also mention that we have a small army of users who look forward to adding tag wikis to these new tags, without actually taking the time to check whether the tags are obvious misspellings or poor tags at best?)
Now, in order to rein in on this matter, I suggest that we raise the reputation required to create new tags to 1500 reputation from the current 300 reputation, while keeping the retag privilege at 500 reputation.
This suggestion is not groundbreaking given it already works that way in Stack Overflow, requiring 1500 reputation to create new tags.
While I understand that having lower reputation for creating new tags might work better for smaller sites who don't have enough personnel or not many other problems, I think for sites of our scale, it doesn't work out well. Especially when getting rid of these tags require Stack Exchange team interaction as the mods don't have the necessary tools to nuke the tags.