I think the flexibility is the key here. Things should be set up to be either completely minimalist - or for more options.
As an example, on the simplest end, it could be almost like a simple poll:
Title (example: Your Review of the Weather today)
Options: (example: Great, Mediocre, Horrible)
Then there would be a number of options that the admin could be in such as the date, the username, etc...
On the slightly more complicated end, there is the stove and fireplace rating system I mentioned in my other post above.
I have another simply "rating" system on my site that I did in my CMS - really more of a "reporting" option as opposed to a review. In this case, they are putting in the price of fuel (wood pellets, for example). In many ways it is similar - a few entries and drop downs, etc. http://www.hearth.com/econtent/index.php/fuels/viewwoodpellets/
As important as the input is the output! A review system should be able to have the output searchable in various ways. For instance, all products with a 5 star rating in this one category, all products of this type, all of this brand name, etc.
Lots of possibilities here - I think the key is sometimes separating the info from forums. That is, although it will be in the XF db and with the logins, etc. it should be able to be set up in completely different templates.