Unlocking all decks in Magic DotP 2014

There are two ways of getting cards unlocked in the standard M:TG Planeswalker games - you either play through the single player game multiple times with each of the decks, which will eventually unlock everything, or you can pay for the deck unlocks. You can get the full listings of all the cards in the decks that come with the game at http://www.wizards.com/Magic/Digital/DuelsOfThePlanesWalkers.aspx?x=mtg/digital/d12/decks - this will show you not only the unlockables for the standard decks but also the expansion decks and scheme decks that are used in multiplayer.

The rest of the DLC out there (The Deck Packs and the Expansion Pack) are totally optional, but you can only get their decks and cards by purchasing them - they are not able to be earned simply by beating the game. Pack 1 adds "March To War" (a R/W Soldier deck) and "Ghoulkeeper" (a black Zombie deck). Pack 2 nets you "Dark Heavens" (Black & White, angels & demons) and "Forest's Fury" (a Green stompy deck). The expansion pack will get you three new decks in addition to new cards for the old decks.

The opponents you are facing are special Encounters in the campaign. When you did your first encounter, there should have been a popup telling you that Encounters are specifically designed this way in order to help you improve your skill at facing specific strategies. These matches are specifically labeled with Encounter in front of the name of the match, like Encounter: Disturbing the Nest and Encounter: Face of the Ghoulcaller.

Encounters will always play out the exact same way. For example, during the Encounter Stampede!, the AI will always play a Willow Elf on Turn 1, followed by a Runeclaw Bear on Turn 2, and so on.

Encounters also, as you have noted, have decks that are exempt from the limit of 4 of the same type of card.

The Planeswalkers you face in the campaign and other opponents, like Odric, have decks that are subject to the 4 card rule. Note that the game with them will also be different each time you play, in contrast to the Encounters, which always play with the same order, use the same strategies and same cards.