Definitely VALUE - I just had a guest who is a frequent hotel traveler. He stayed at 400+ hotels. He gave us a perfect rating for everything but value. When we asked for a reason or reservation, he told us he is comparing us to Courtyard and Westins.
Value is such a totally subjective aspect, even for a same product, it could mean different value to different people to their income level, life experience, personal style and preference. Out of all the five aspects Airbnb asked Guests to review Hosts, Value is the most subjective, unspecific and hard to define aspect.
In Hosts review of Guests however, hosts are not given any review aspect like this. Hosts are not given an Overall review as well, but are asked to review on Cleanness, Communication and Following House Rules.
Airbnb should keep a fair and neutral balance between Hosts and Guests, as they are equally important to Airbnb business model.