What if, after you answered a practice question, you were able to see the percentage of Khan Academy users who answered that question correctly? I think this would be an interesting way of being able to see where you stand compared to other students, and you would be able to judge how difficult a particular question is supposed to be.
I got this idea from chess.com's puzzle section. Chess.com has a collection of chess puzzles. The user's job is to find the best chess move (or series of moves) in the presented position on the digital board. After the user completes the puzzle, the site displays the percentage of users who solved the puzzle correctly on the first try. I think this would be an interesting thing to have on Khan Academy's SAT prep.
What do you guys think?