On the deck page, when building a deck:

If I'm looking at my own deck, the add card text-entry should just be there. I shouldn't have to click a menu item to make it appear. Less clicks = good UI (until it gets too crowded, but you're not there yet)

It should let you know if you've added a card that's already in the deck. I was listing a deck and I entered part by memory and part looking at the deck and it was a real pain dealing with duplicates that shouldn't have been there.

There should be a way to manually designate what format a deck is built for (Standard, Legacy, Commander etc.) or if that exists it should be easier to find. It would be nice to post a work-in-progress EDH deck that doesn't have 100 cards yet and have it marked as EDH.

Speaking of EDH, there should be a way to highlight and set apart the general.

Forums:

Please divide the deckbuilding forum into sub-categories for different formats.

Just minor suggestions, love the site. Y'all are doing great work!