Operation: Anchorage is a 4-5 hour campaign (we got through it a bit quicker) that calls the game's FPS element into play more than we'd anticipated. In fact, the scenario is a linear one in the vein of Call of Duty and is very focused on action. Sure, the trademark V.A.T.S. targeting system is still present, but, most of the time, we found ourselves playing in a purely run-and-gun mindset. Still, there's a depth of story and uncanny attention to detail that very much manages to keep this DLC rooted in the Fallout universe.
Be prepared for a far more constrained experience than the (literal) sandbox of the Capitol Wasteland.
The most immediately obvious change: you start out with your "real world" stats, but only a silenced 10mm pistol and Stealth Boy to your name. (Thus you're prodded to play it quiet and cool from the get-go.) The second major change is obvious once you've taken down your first enemy: you can't loot their corpses because they vanish in a flurry of VR "de-rezzing." This leads to the third substantial difference from Wasteland combat: you find weapons -- such as the glorious new Gauss rifle -- at set intervals in the "levels," along with health and ammo recharge stations. Think Left 4 Dead. There are also holotapes and intel cases to find for completests (and those who wish to delve deeper into the scenario's story).
The change in rules and level structure -- even the reliance on reaching health/ammo drops -- creates an experience that is more tense than almost anything in Fallout 3's main game. Although it's probably going to feel a lot less so for those who already have high-level characters. That's because all of your perks, skills, and stats carry over. If you're a dead-aim in the Wasteland, the VR enemies will be much easier to head shot. On the other hand, if you play the campaign early on in your Fallout 3 experience, it's going to prove very tough.
Considering that those most likely to buy the DLC are players who've already completed, well, everything in the main game, Operation: Anchorage could certainly offer a bigger challenge. Still, it's a very exciting -- and surprising -- addition to one of 2008's best games, doing a great job of extending its life and keeping things fresh. If you love Fallout 3, you'll really enjoy this addition to the 'verse and the gameplay mods it brings. Just be prepared for a far more constrained experience than the (literal) sandbox of the Capitol Wasteland.