While I think that MB is a really well structured and thought-out database and the input methods are for the most part great, these are the three biggest gripes I have as a MB editor and user who uses the database for tagging:
Original release date
It depends on an exisiting release within a release group. Really, that is so bad that I always use Discogs to tag first. The original release date MUST be a release group value. I know, the transition would be difficult, but that is the one big NO of MB.
Matching release group when importing from other sites
For common words /short phrases this sometimes results in an endless going through results. I don’t know why the list of results is so inefficient sometimes, it’s almost as if album title is not paired with artist for a search. Direct search is no better, just a different sort of annoyance
Catalogue number should (also) be medium based
Sometimes releases have several catalogue numbers, one for each disc of a set. Some editors even enter them as XXX-790~5, when obviously they should be XXX-790, XXX-791, etc. In any case, all numbers get attached to the whole release, when they should be associated consecutively to each of the six media in this example.