Scalability, how big could it get without causing problems, can it handle multi user operations? etc.
Security - have you implemented access controls? and how good are they? are the access controls multi-level eg? supervisor level, user level etc. Have you implemented audit functionality?
Documentation and maintainability - if you walk away how will someone else be able to pick it up and maintain or extend it?
Have you performed a soak test to see if you're leaking memory or handles?
Missing functionality - anything you would have implemented but didn't have the time?