The length of my 5 (so far) reviews has varied quite considerably, depending on how much the was to say that I thought was worth saying. Space on this site isn't on the ration, so if a reviewer has plenty of interesting stuff to say, I don't mind if the review is a long one.
While I agree that effusive reviews in the style of a Mills and Boon novel are a waste of the reader's time, so are brief (and often badly-written) reviews which fail to communicate enough information to give a clear idea of the girl or the punt. This is particularly unsatisfactory with negative reviews which simply say something like "This woman is a cunt and bitch. Avoid her".
These just come across as rants.
Some people are fluent writers, and if their reviews are long, they also tend to be an easy read. Conversely, some people post reviews which are about as inviting to read as a tax return.
So, I think it's "horses for courses". I, for one, would not like to see a limit put on the word count.