If you want a challanging 2v2 with A.I. you should try with insane difficulty A.I. That will make the MP campaign last a lot longer with the A.I being more aggressive and strategic then. (though the A.I. still isn't very tough in my opinion then. its a GREAT improvement, but all it needs is a dragon and it would be a real challenge. still fun to fight against at this time though.) Also, if you go up against other people in multiplayer campaign then it probably will last a whole lot longer as well. With two or more skilled players facing off against each other the battle could last for hours. (look at twitch tv's master vs apprentice on Larian's channel.)

http://www.twitch.tv/larianstudios

And, as Ravenhoff said, the Single player campaign will have diplomacy and choices and consequences and much more. so there will be much more depth then. Also, notice that there is "custom campaign" greyed out on the main menu. That likely means that we will have the option to play the "multiplayer campaign" with single player campaign features mixed in, which would be AWESOME!

Also, there will be many more maps, skirmish and MPC, and that can add a lot of variety and strategy to the game. A while ago on a previous patch there were a list of the skirmish maps but you couldn't access them, but the list was HUGE! No preview map either.

I am looking forward to the release of the game.