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Reviews shouldn't require a body
We are saying the same thing. That said, you can still approach this problem in one of two ways: the first is to do as you suggest and simplify the review form to one field. The second is a bit more complicated: make your form smarter and make only one field required (it doesn't matter which one) then store the one that is filled out as either the subject or body; whichever is more canonical in your system.
I think your character limit is fine. It might need lowering a little but I've seen some really shitty "reviews" which are just a single letter or a smiley/frowny face and of course this sort of content should be discouraged.