When it comes to studying, sometimes in your sets there are some 'plug and chug' type questions that are considered 'easy' and have a high percentage of students getting them right.  A basic appendicitis presentation, for example.  I think a cool feature would be to be able to sort questions by difficulty (based on percentage who answer right).  You can review your questions based on difficulty already (easy, medium, hard).  Perhaps you want to knock out all the 'easy' ones first as a warm up.  Or you only want to do 'hard' questions because you want to focus your studying on the perceived 'harder' topics/questions.  I think this would make the product more valuable to those who want to really narrow down what they're studying.

Allow unhighlighting of the last highlight command with a single click instead of having to highlight the entire highlighted section to unhighlight. 

I reset my score because I finished the questions and wanted to start over, but I like having my marked questions to make specific exams from... if all questions could be reset to new then being about to do "new" questions while making "marked" exams so I don't repeat questions I just saw 

I wish there were more pictures in the explanations. For example, there are so many questions where the explanation will include a description of histology, but it would be really nice to just have that picture there as well. 

Each day send a short reminder of a wrong answer for repetitive learning

It would be nice if we could select all incorrect questions both by system and difficulty.  That way we can see where there is a pattern of specific topics/concepts that we are weak in.  It can be very frustrating when reviewing and you keep making new notes on a topic you already made notes on.

I think having the exam have the same color scheme as the COMAT would be helpful. UWorld's interface looks almost identical to the Step Exam, and even Combank has the interface mirror what COMLEX or a COMAT looks like.

Test day countdown / number of questions you need to do daily to finish Qbank before test.