Given that less than 10% of posts were given community tags and you can still offer up 2 tags at time of post, honestly in this regard, not much has changed. I realize that the second tag can be changed by the community, but unless that number increases (which it may) I don't think you'll find much of a difference.
I can see one of two things happening.
If community tags increase in popularity, then people will have to post their initial tags with the idea of, hmm, which of these tags am I willing to sacrifice?
That, or community tags will decrease in popularity as a result of this feature, because people will feel they are infringing on the submitter's original choice in tags.
Before, when you posted you weren't affecting the original post at all, now you can directly screw with people. Vindictive users could abuse this, bots could abuse this, etc.