A few thoughts -
I'll vote a review not helpful if it's really short and doesn't add anything beyond the "this course is awesome" or "this course stinks" sentiment. The less reviews like this the rest of us have to wade through the better. I've also voted not helpful on just a couple reviews of courses I know well that I think are way off base. And not that the ratings were too low, but rather too high. I'll just not vote if the review is anything more than short and pointless (in my view), and I'm quick to vote helpful if I think the review is good, whether it's a course I've played, or a course I'm hoping to play. I don't generally vote at all on the rare occassion I read a review that doesn't fall into one of these two categories.
While I personally am a fan of longer reviews, I try to keep mine from being too long simply because I want more people to read them. I fear that the longer the review the less people are going to work through the whole thing. Being succinct is a real skill in writing, though, and reviews that are both short and helpful are out there, but rare, IMHO. The balance of thorough, adding some information of value that isn't already out there, and still being succinct, that's a tough thing to achieve.
I'm alway a bit surprised and amused by how seriously some people take the thumbs down. This is all subjective and all opinion - even the best reviews are going to provoke disagreement from somewhere. Thumbs down are unavoidable and hey, at least people are reading.
People are quick to attribute thumbs down to "haters" or HBB, but hey, some reviews aren't so good too.