I think putting the good before the bad is best. I think the Wombats form does it well with their wording: "What could we do better". It puts people in a constructive frame of mind and encourages them not to say: "The kitchen was a mess" but "It would be better if you arranged the shelves differently".
I personally, writing a review, prefer to write it as a whole, eg "The party atmosphere in the common room was great, although sleeping was difficult when guests continued banging the doors". I don't like to be told how to write it. Splitting the good from the bad is reasonable, but I can't go with the BUG site's: "What was best about this hostel" and "What was worst about this hostel". I just am dumbstruck at the second question, as it's not framed within my concept at all. "Worst" sounds too drastic. And Hostelbookers' "What did you like about this hostel" leads to a set of almost valueless reviews unless people "disobey" the question and add something negative if they need to.
Incidentally, I refused to do Hostelbookers' review not just because of that, but because they forced me to answer Yes or No to two questions that I neither knew nor cared about. I'd love to give them a rating for the hostel as their ratings are valuable, but when it comes to: "Did the description of the hostel on Hostelbookers match what you found -- compulsory answer" I haven't the faintest idea, having 2 weeks before investigated several hostels on numerous sites!!