Well, it just plays really well !
I started by playing the demo. The tutorial is ok and wasn't to hard to beat. It teaches you the basics but doesn't really teach you any tactics because that's what this game is all about and that's why you are going to get your ass kicked at first.
Once I unlocked the full game I started playing the hard campaign and man it's hard. The basic concept is that when you can build a lign of defense on the board, your units support each other making them stronger in fights. So basically, what you want to to is break the enemy defense line, find the weak spot and destroy it. Once it's started you can domino a lot of damage on him.
To take out that weak spot you will need to concentrate a lot of power and that's where the cards and the units come together. For example, you can group a lot of artillery around the attacking unit and then play a card to add +2 attack per artillery unit in the fight. Another strategy is to weaken a unit dug in a bunker down to 1HP and then attack him and play the kill commander card (1 DMG). Even if he was supposed to fire first (dug in a bunker) he is dead before the war began.
So anyway, I am currently halfway through the campaign and damn it just plays really well. I haven't started multiplayer yet but I can't wait (I need to finish unlocking more cards before I start building my MP deck).
Overall, cool game, really cool game.