I have been using Moodle quiz since 6 years, It has a great question editor, a great exam module and great import/export function. we built an exam center with it, made more than one hundred successful high stakes e-exams with moodle 1.9 and it runs flawless HOWEVER...
Although these great capabilities; moodle fall short on question tagging and metadata, in other words... if there are customizable extra fields for each question that I can add to better store and retrieve the question, combined with a search function... This could be Great.
And if the categories can be organized in tree like structure and has themselves some metadata to describe the categories and these categories are sharable between courses in a way much easier than the current implementation.
A central bank in the moodle installation will allow better storage, sharing and exchange of questions.