If you start the title of a question with a construction like "[tag]", you get a red box telling you that:
Title contains a [tag] prefix; please use the tag field to enter tags instead.
And so, I can't title this post "[tag] prohibition in titles on meta sites is inappropriate". A travesty!
This blocking dialog is certainly appropriate on all main sites, but is not appropriate on meta sites (child and Meta.SE alike), where "[tag]" may legitimately be used at the beginning of a post title to refer to the main-site tag "[tag]". Enclosing tag names in square brackets is a somewhat-established convention (I guess because that's how you search for them), which is why I tried to do that.
Yes, I know I can work around this by rephrasing my title so that the tag name is not at the beginning of the title, or by not enclosing the tag name in brackets. Still should be changed, in my opinion.