Are you ever frustrated that a practice CAT doesn't have a certain feature or behaves a certain way?

I am a grad student in the bay area and I am currently working on designing and programming a high-end GMAT CAT practice exam. While a major thrust of the project is using a plethora of new technologies (won't bore you with the details), my question for you: what features would you want in a practice exam? what do you wish other exams had?

please think outside the box as I've got a fair amount of experience and the new back-end architecture I am working on as part of my studies is very powerful. Your help would be greatly appreciated as it will substantially improve my final deliverable. Please feel free to PM or email me (stanford2010 at live.com). Thanks so much!