Finished Act 1 on Nightmare mode last night. One thing I can certainly say, they are amping the game up for a more savvy gamer crowd. In D1 and D2, in the harder modes you would just make sure you were sufficient level for the boss, and carefully watch your health resources and you were going to win. Not in D3. Endboss Balail throws out some abilities that make you move and actually employ tactics in order to beat him. Took me three attempts to bring him down. I'll wager in the Hell and Inferno modes, you actually have to research tactics on theorycrafting sites. That's pretty cool, actually.