gleaming plinth
If you have multiple variants of a deck, it can be easy to forget which deck has specific cards. Plenty of times I've found myself in a situation where I can't remember if a specific game-winning card might be in this deck (such as To the Last Man while my opponent has half his deck on the field). It can also hurt if you can't quite remember how many of a specific card you have left to draw.
Hovering over your deck should, after perhaps a short delay, display the cards remaining in the deck, in addition to the current number of cards left in it.
harsh radish
elfin crown