There are 2 things I would like to see added to Hearthstone either in the Curse of Naxxramas expansion or in some future expansion or update.
1) A creature-spell balance indicator. The game currently has a good mana-curve indicator, but I frequently ended up with deck that either had good spell combos, but lacked creatures to finish the opponent off, or had great creatures, but lacked the spells to counter what the opponent had. Adding a simple indicator that would show you how many creatures and how many spells you have in your deck would really help out, especially for newer players who don't want to use advices provided by the game when creating their decks.
2) The ability to create more than 1 deck for a particular class. This is really simple. I have a decent Mage secrets deck, but I also want to try, say, a spellpower upgrade Mage deck. Or I have Warrior deck with a lot of charge in it, but I would also like to try, say, I weapons deck. And I would like to avoid having to dump all cards I have in my Mage deck to substitute them with other ones and then having to repeat the process if I suddenly desire to have my previous deck back. Having multiple decks of one class is just convenient.