Starting with, arguably, the least interesting card, Pumpkin Peasant is an average-looking card with underwhelming stats, and Lifesteal doesn't really compensate for that. However, the utility of being able to use the card either offensively or defensively, especially with hand-buffs, could prove useful in certain situations.
Next one up is the Phantom Militia, which sounds like a good name for a rock band. This card introduces a new mechanic: Echo. Once you play the card, it will create a copy of itself in your hand and, provided you have enough mana, allow you to play it again on the same turn. Under normal circumstances, up to 3 copies can be played on the same turn for 9 mana. Many have already pointed out this card's synergy with Warlock's class card Summoning Portal, which would allow up to 6 Phantom Militia to be played on turn 10. Think of it as a Spreading Plague for Warlock in that case.
Militia Commander is the only class card revealed so far for Warrior, and it also features a somewhat new mechanic. The Rush mechanic is your typical Charge, except it can't attack heroes on the turn it's played. This has already been seen in the past with cards like Icehowl and Charged Devilsaur. The battlecry, however, effectively makes this into a 5/5 charger, and if you attack with it on turn 4, you will often be left with a 2/2 minion, which is a pretty damn good deal. It's essentially a better Bomb Squad.
Last, definitely not least, Azalina Soulthief is a highly understated 7 mana legendary neutral minion with an equally powerful effect. It has been compared to a Paladin class card Divine Favor, which draws cards until you have as many as your opponent and has been included in aggro decks for the longest time. This, in essence, has a similar effect, with the exception that you are taking your opponent's cards and, hence, if you are playing against a combo deck, the cards might not be as useful to you. Regardless, this is an exciting card and one you will see a lot during the first weeks of the expansion.
The precise release date for The Witchwood is not yet known, but it is slated for a mid-April launch, and we still have 129 new cards coming up. Stay tuned to GameSkinny for more hearthstone and The Witchwood info as it develops.