It's a word coined by pg that's sort of a term-of-art for the depressing tendency of the top HN comment to be something which pooh-poohs the company/topic/idea in the submission, doesn't add anything to the discussion, but (crucially) is worded convincingly enough to not just end up on the bottom of the thread.
e.g. "Lol that suckz because Google can steal ur passwerdz!" would not end up at the top of the thread. We wouldn't be terribly worried if someone actually said "Wait, I just audited NativeClient and it turns out you can achieve arbitrary code execution. Maybe you should avoid this software." The danger zone is comments which sound like the second but, on reflection, only tell you about as much as the first.
Ironically the concept of a "middlebrow dismissal" seems to me to enable what it condemns--e.g. HN'ers can now just post "hey that's a middlebrow dismissal" instead of a detailed statement of why a particular response falls short.
Basically I think pg made it worse by giving it a catchy name.
This makes sense, but it makes me think that (in the interest of having a good top post) you should have top-leveled your in-depth comment rather than responding to olalonde's as you did. Your comment then could have (and probably would have) been voted up to the top.