As someone who's recently started importing their collection into deckbox.org from excel, I must say that it is a pain. As I'm sure the creators here realize that allowing other importing options (from .xsl files, for example) would help alleviate this, let me make another suggestion:

For cards that have only ever been printed as a single version from a single set, automatically 'correct' their details to incorporate that set.

As promotional cards don't seem to be included in the database here, I'm unsure how you'd approach cards that, while available as promos, are only available in 1 regular set. You could reference a separate database of all the promos to determine which cards to ignore, or just ignore promos altogether. If the latter, you'd want to enable this as a an update-option, rather than as a persisting rule. With approximately 9500 cards (3/4 of all distinct cards) being 'unique' with only 1 printing of each, this could potentially 'fill in' a majority of someone's missing set info.

BetweenWalls wrote: For cards that have only ever been printed as a single version from a single set, automatically 'correct' their details to incorporate that set.

This would be really really awesome to have!