What the heck is it? I seriously can't tell from the front page.
That might be a little too negative, my initial impression of this is that whatever small amount of utility it may actually give just isn't that interesting or different from what we already use.
But after clicking around I worked it out. It was an elitist (and far less useful) version of Quora. Or was it simply a polished discussion board. Then I clicked the About Branch button and realised it was more about the Twitter integration than anything.
IMHO: Try and clarify what this is on the home page. "A new way to talk to each other." is contrived, meaningless and simply not true. It's not new, it's better.
Quora is for monologues. Branch is for dialogues.
Quora gives your full attention to an answer. It collapses the comments. It lets you downvote unpopular opinions.
Branch limits the most that anyone can say at a time (to something like 800 characters--which is enough for an explanation but not enough for a tirade). And it gives equal weight to each comment that someone makes.
They give you a way to publish comments for any public URL on the web. And those comments live on their site in a comment thread they call a "branch."
I wonder how close I am. Clicking the "Let's get started" button now...
Hmm, I think I was pretty close. Branches, or comment threads, can be closed. Interesting. So they added a life span to a discussion, vs keeping it open for more comments. I wonder why. To preserve the quality of the initial discussion? Hmm.