I have a review-driven site as well, RateMyStudentRental.com, so here are a few observations...

-People review for all sorts of reasons, but the largest reason students write reviews on RMSR seems to be simply to "give back to the community." In other words, they just like helping. However, keep in mind that my site helps students find suitable housing, so compared to product review sites (like Amazon), there's probably much more of a charitable motivation for RMSR.

-People like sharing and helping, but not that much. If the review form requires too much info, they'll just say "ahh, fuck it," and leave. RMSR's review form is still WAY too long if you ask me, but I just haven't had time to change it. That actually leads me to a piece of advice: try to get your review form nearly perfect from the outset. If you have a semantic site that arranges data in a meaningful way from the reviews, the review form becomes difficult to change, because every change requires some data migration of all your existing data to the new common format. Not to mention the extra logic you have to add to the site. E.g. if you add a criteria on which to rate, do all the existing reviews get a zero? Obviously not, so now you have to add in some if-statement for "N/A" and get fancy with your AVERAGE calculations.

-When we started, there was a lot of fear (some from us, mostly from landlords and school officials that were on board) that the site would just be a landlord-bashing playground for tenants. Thankfully, rather than site around and try to invent rules, incentives, policies, etc, we decided to simply address it as it became a problem. This was one of the best decisions we ever made, because here we are 3 years later, and it still hasn't become a problem. Out of a 4.0 scale (like report card grades), the average rating for rentals on the site is like a 3.5 or something. This brings up another interesting point: they say reviews tend to display the extreme ends of the spectrum (either really satisfied or really dissatisfied). FWIW RMSR probably still has way more A reviews than C, D, and F reviews combined.

-At one point, I tried integrating some game mechanics, by giving badges and stuff for the number of reviews a student has written. But we ended up getting rid of it mainly because it added complexity and no one seemed to care. No one cared most likely because RMSR is a site that students use only a couple times a year. If you only use a site once every 6 months, why does it really matter? Note that I could be totally off on this, because our "badge" implementation was half-assed at best, so perhaps I just didn't try hard enough.

-To motivate students to write reviews when we first started, I wrote a promotion engine, where if students from a certain area of schools signed up and wrote a review, they'd get a print-out coupon for a free coffee from a local coffee shop. In return, the review would get the coffee shop's logo placed on the review for all eternity, which would link to the coffee shop's website. I wrote this such that it could work with any local business. It worked really well, but by far the hardest part is getting businesses to give stuff away that is actually worth the students' time.

-Some students leave reviews because their landlords asked them to, so that's perhaps a motivator specific to us. I've even heard reports of landlords waiving rent late fees in return for writing reviews (though I haven't seen any indication that it affects the quality of the review or rating, which I was afraid of when I first heard of it).