When you view a deck, it tells you 'This is a complete {colour} {format} deck'.. where 'format' is 'vintage', 'legacy', 'standard' etc...
What I'd find really useful, is a feature where, when viewing your deck, you can choose a deck format, and it can show a list, or maybe just highlight the cards that are invalid for the given deck format?

For instance, I have some 'vintage' decks, and I'd like to know at a glance which chards violate qualification for standard play? .. Same for legacy...

Perhaps it could be as simple as a column in the listing, and then you could sort by that column.

Anyone else find that useful?