it'd be more reasonable if the frequency of a page being the question was proportional to the number of links pointing to it 
Some minor bug.
The question was In mathematics, a ??? is a certain kind of subalgebra of a Lie algebra. The subalgebra of a Lie algebra is a Cartan subalgegra if is an abelian subalgebra, with the following property of its adjoint representation: the weight eigenspaces of restricted to diagonalize the representation, and the eigenspac ...
Probably taken from the page Cartan subalgebra.
The phrase "Cartan subalgebra" appears in the text again, and only the first occurance is hiddn by the ???.
Second point I wanted to make was that wikipedia has subportals like http://en.wikipedia.org/wiki/Wikipedia:Wikiportal/Mathematics and catagories listed in the Wikiportals
This can allow you to restrict the quizes to a single subdomain/topic.

Both questions are the core issues I realized I need to work on: filtering to more easy questions (e.g. by PageRank – the more popular, the easier) and allowing for categories (which would then also make it easier, because the player knows the subject). Thanks to you two. 
If it's very hard – it's not a trivia! 
You might also keep track of the win/loss ratio; then, you could walk people into an ideal level of difficulty in which they have a 50% win/loss ratio. (If they win, it gets slightly harder; if they lose, easier.)
Of course, then we'd all know how smart we weren't, and that wouln't be any fun.
Good work! 
On second thought, has anyone actually gotten one right? 
Yeah, I had "Gandalf" right once... but I'll definitely work on the level of difficulty and/ or find a way to choose a category for the player. 
I got two right. One was a date in a series of dates (it was obvious)... and the other was the word Chasis. I had several partially correct answers (e.g. Wilma!= Wilma Flintstone) as well.
I think restricting by category (ala Trivial Pursuit) would be very nice. Giving credit or a retry for partial answers would be gratious as well. I had no clue on questions with foreign language answers.
Honestly, the premise is wonderful and almost works. It could be very fun with a win/loss high score list (assuming people don't cheat with another browser window) 