To understand the tonal difference between Alan Wake and new Alan Wake's American Nightmare (XBLA), you need look no further than the title card for both games. In the original, you can hardly see Alan at all; the important visuals are the oppressive shadows created by his tree-filled environment and the flashlight that Alan uses to fight the forces of darkness. In American Nightmare, Alan stands tall atop the red Arizona rocks, nailgun in hand, looking more like an ad for Craftsman tools than a self-absorbed writer who may or may not be insane. If Alan Wake was about exploring mood and creating tension, American Nightmare is about shrugging off the big questions and just shooting everything in sight. It's a slightly different direction for the franchise, but a fun one.
It's ok if you haven't played the original Alan Wake or any of its DLC - American Nightmare stands completely on its own - though much if it will likely be confusing to anyone not familiar with Alan's previous dealings with his dark alter ego, Mr. Scratch. Alan attempts to catch up new players by offering bits of explanation to some of the people he meets, but his plan to fight the physical manifestation of his dark impulses by rewriting reality is a tough concept to convey with just a few lines of dialog. American Nightmare takes place in Night Springs, a Twilight Zone-esque show that was Alan's first gig as a paid writer. To escape, Alan will have to bend reality, which apparently involves listening to Kasabian and doing lots of shooting.
Alan will have to fight off legions of the Taken as he tries to bend reality to his will. Once human, the Taken are impervious to harm until Alan uses a light source to burn away their protective shroud of darkness. Veterans of Alan Wake will know to save up flare gun ammo and flash bang grenades for big swarms of enemies, and rely on Alan's standard flashlight to render Taken vulnerable to one of the many guns Alan can find lying around the desert. The combat is well-paced and becomes steadily more challenging as the game progresses, but short of a few moments where you have to hold your ground against a tide of Taken, it's never really all that difficult. The environments are peppered with ammo boxes whose contents replenish after just a few minutes, so you're never more than a few steps away from topping off your arsenal, or in real danger of your gun going dry.
Like Alan Wake, American Nightmare has you hunting for manuscript pages, but the search isn't quite as satisfying this time around. In Alan Wake, the pages were from a novel that Alan didn't remember writing and that seemed to spookily mirror events unfolding around him. Finding and reading them helped you share the experience with Alan, as you discovered their narrative together. The pages in American Nightmare have a similar story to tell, but their main function is to unlock weapons cases left lying around the desert. The cases, which contain tasty morsels like submachine guns and sawed-off shotguns, require specific numbers of pages to unlock, so you'll be scouring the countryside in the hopes of leveling up your arsenal, not delving further into a mysterious manuscript.