If the advantage of tags was that new users could discover more content via them and thereby could build the amount of people/tags they follow, then this has just been drastically improved imo. I can now go to #science and find posts I think are interesting and then follow science.user and the user that posted it. I may find that the user posts other content I dislike and can remain following their science tag. There are plenty of scenarios like this that exist. But if your main concern is the loss of the second tag, my guess (and this is only an experiment) is that you'll find it's largely unchanged. I look forward to your thoughts in a week or so. Definitely.but this essentially went from miles ahead of reddit to only as far as you can throw a baseball ahead of reddit.
If the worth of the site by your estimation, hinges upon the ability to offer two tags, you're in luck you still can. 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.