It's still structured, but not so much that you can't play around with those initial 60 cards in each deck. While lands are set in stone, any other card in that base deck list can be swapped out for another -- outside of the restriction that a deck must be at least 60 cards, you're free to mingle cards in and out that fit the parameters of that deck (e.g. color and creature themes).
The sad news is that none of your content from the previous game can be loaded into Duels of the Planeswalkers 2012. In fact, you'll see a lot of the Planeswalkers from the previous game in 2012, but their decks won't be the same -- minor tweaks have been made here and there, in line with the new 2012 core set that Wizards of the Coast is launching in July. Deck content will mostly be in line with that.
The mode being demoed to me was Archenemy, a purposefully unbalanced game type where one AI opponent has double the life and pulls from a deck of "scheme" cards -- a free-to-play modifier that effects each turn in a different way. Facing the archenemy are three mages: the player and two teammates. These teammates can be AI, or local or online human players.
Archenemy is rough. You're out-classed almost every step of the way and it can be infuriating when the Archenemy keeps ripping scheme cards that dole out tons of damage. Compounding that frustration is when an AI ally has a card that could severely turn the tides for your team, but you have no way to tell them to use it and instead they play something that's idiotic.
Duels of the Planeswalkers 2012 looks to be an improvement over last year's game in every way. There's a much lengthier single-player campaign this time around, support for online co-op, a new Archenemy multiplayer mode, a cleaner, more vibrant UI, all running on a tweaked version of the first game's engine, so the controls should be familiar. These are iterative enhancements, for sure, but ones that show Stainless Games has made concerted efforts to respond to community feedback and offer a more robust, diverse experience.