I'm certainly not alone in using your service not only to hold decks that I currently have, but to build decks that I think are interesting. That being said, I think it would be nice if there was a way to add a check box indicating that "Deck X" is currently active and as such, the cards will not show in your inventory for purposes of building another deck.

For instance, I have a budget-ish Mono Black deck which uses Rakdos Shred-Freak in it: http://i.imgur.com/qpEHaCY.png
As you can see, I have four in my inventory and they're all used in this deck. However because I saw a deck that looked interesting, I imported it: http://i.imgur.com/2lBDObC.png and as you can see, it essentially wants me to take my cards from my black deck and put them in this deck.

What I would like is if I could mark my black deck as active so that when I went to the other deck, it would show [perhaps even in another column or something] that I would need to buy these cards if I wanted to make this deck while maintaining my other deck. Also, this would be beneficial to people -like myself- who share a library with their significant other, who also plays and builds decks.[/perhaps even in another column or something]

^THIS ALL THE WAY!!!!!

I would also love this feature.

I have a lot of old decks and the ability to archive some of them (without manually exporting) would be highly appreciated.

I would love this as well.

I version my decks, so I constantly have older versions of the same deck that are always lying around, and it would be great to get those marked as inactive, so that they don't necessarily show up in the Decks listing for each card.

Versioning would also be a great addition to deckbox. I now type in my changes in the comments but it's a bit cumbersome. A more automated functionality would be very welcome.