There is little control over what is quizzed.
If you could allow the user to check words that they want to be quizzed on, that would be a great help.
If you showed a multiple choice of 5 words on the left and 5 meanings on the right, then have user match correctly, that would be nice.
We have planned to address the first part of this issue: http://lexicum.uservoice.com/forums/239781-user-requests/suggestions/5654587-let-me-define-subsets-of-my-wordlists-for-me-to-fo
The word-matching suggested in the second part is an useful exercise that we are considering as a possible feature at a later stage.