I am not entirely opposed to hashtags, and I get how they will be useful in letting me curate the posts I see. But left unchecked, they are going to splinter the community. Things that would have been posted in Flood now may or may not be posted with the #flood tag; they might be posted as #offtopic, #funny, #politics, #tacos, or #whateverthesubjectofthepostis. The community is going to be broken up into a seething, chaotic mass without any kind of organization.
For example, I want to keep track of the discussion about the new forum format. But I can't just go to one place for that; instead, I have to follow seven different-yet-related hashtags (#feedback, #bungie, #bnext, #bungiedotnext, #BungieNext, #hashtags, #Septagon) some of which are literally synonyms for each other. And that doesn't even take into account all of the posts with misspelled hashtags that will never get noticed. I believe that this is going to be a serious issue, and will change the forums from a tight-knit (if occasionally dysfunctional) community to a disorganized mess.
However, I also believe that there is an easy fix. All you have to do is require a post be tagged with at least one of seven hashtags (Feedback, Destiny, Halo, Flood, Gaming, Universe, Septagon), after which the poster can go on and add whatever hashtags he or she desires. I will still be able to curate the posts I see exactly as well as I can now, but there will also be a layer of structure that will hold the community together. Do I want to browse through Flood? Then I'll go to #Flood, confident that I'm not missing anything. What about Halo 2? I'll search #Halo2, and know that if I don't find what I'm looking for, I can always check #Halo (which would contain all Halo-related posts).
I like that Bungie is looking to the future, and I agree that the old-fashioned forum format was out of date, but we have to be careful in how we change it. I can't think of a more tight-knit community this size, and I don't want to see it broken apart.