Prior to the release of Naxxramas, Midrange Druid was a staple of the metagame. This was a very popular deck on ladder and in the tournament scene because it had no bad matchups. No matter what you were up against, Druid had a chance to take it down. The consistency of this deck is why I brought it as one of my decks to IEM Shenzhen last month: it went 3-0 in the finals. Here is a video I recorded there with HyperX outlining some of the strength's of my build.
Choosing your mulligans with Druid are pretty straight forward. A lot of Druid's power comes from the fact that their cards are all universally good and flexible. As such, you can mulligan more aggressively for Innervate and Wild Growth. These cards are important to all of your match ups because -- since your average card quality is so good -- you just need to be able to cast them to win.
If you have a read on what your opponent is playing, there are some additional cards to look for beyond the ramp spells. Against the aggro decks, Harvest Golem and Keeper of the Grove are key players. Harvest Golem is good because he comes down early and will often take out two minions. Keeper of the Grove does the same for even better. Keeper has the option to kill something immediately, and he can trade for two 3/2s in the Zoo match up if you have time.
Control has fewer specific cards you need to mulligan for since your game plan in this match up is to overload their answers. That said, The Black Knight can do work as both a thread and an answer.
Looking Towards Naxx
After leaving Naxxramas, many of the dominant deck archetypes are getting a boost. Midrange Druid is no different. I've made some picks for which cards can contribute to this archetype and built some decks around them.
This guy has been getting a lot of hype and for good reason. He starts off as a very solid 5/5 for only five mana. Loatheb is a very powerful card because he can lock out a game if played at the correct time. In this deck, Loatheb is taking the slot of an Argent Commander. Loatheb is brought in to add some extra disruption to the deck. Timing is everything with this card though so don't just run him out. You should aim to time your Loatheb to hit before the big turns of Freeze Mage or Miracle Rogue.
Spectral Knight is another very powerful card which requires an answer from the board. As such, Spectral Knight is a great minion to Taunt and there are three cards in this deck to enable that. A Taunted Spectral Knight is a stop sign for Miracle Rogue, the boogeyman of Hearthstone. With no way to remove this Taunt with spells and no way to attack past it, Miracle Rogue is required to use their limited minions as their only path to victory. To support the Spectral Knights and the minions which give Taunt, there are plenty of other good targets such as Chillwind Yeti.
The More Minions deck is the answer to metagames which are swarming with aggressive decks. This build lowers the curve of your conventional Midrange deck with the inclusion of Haunted Creeper and Sludge Belcher. Both of these cards are good against aggro, but they fill a secondary role in this deck. The Deathrattle of both of these minions leave behind additional minions so I've included double Savage Roar. Savage Roar pulls double duty with your smaller leftover minions in that it can either win a game or help trade up if you are in trouble.