Yeah, thats a big drop off point to go from 98% to less than 50?
I would say as Aylaine did, about 80-85%.
Also have to consider that a large part of wikipedia factual errors are not so much errors as they are opinionated statements. Much like the notorious edit on Halo 3 by Sony saying "Halo 3 will not look that much different than Halo 2, before it was released.
And of course the ever present "Insert immature juvenile edit just because I can" edits that their moderators do an outstanding job of keeping under control considering the sheer mass of wikipedia.
Consider that content wise, Wikipedia is like 6 times the size of a more noted and venerated reference the encyclopedia Britannica. Now granted Im sure 2 of those sixes goes to inane content such as, the entire list of transformers, comic book chars, celebrities, atheletes, ect.
Anyway...I think the biggest reason for hate toward wikipedia is just because educators know how easily it can nullify a large part of the difficulty they put into their syllabi (what is the proper plural of syllabus?) and thus forces the challenge back onto them... well that, and some people are lazy and dont use Furburts technique for using wikipedia and just cite wikipedia outright.