The original Halo borrowed extensively from the short but colorful history of the FPS genre, but it brought together the best elements in one complete package, wrapped around a compelling sci-fi storyline that was cinematic enough to be a movie on its own. I liked it enough to play through the campaign three times, once with a friend on a split-screen, and once through the dreaded Legendary difficulty. However, much like Matrix Reloaded was to the original Matrix, I found the sequel to be a little lacking.
In Halo, the player took control right after Master Chief was thawed out, letting you feel as if you were the one behind the armor, working your way, bullet by plasma blast, to saving all life in the galaxy. Halo 2 begins with the trial of the Covenant Elite who failed to control the original Halo, yet knowing nothing else about him, it's hard to care about his fate. It's also hard to endure yet more cut-scenes while waiting for the first firefight to begin. The overall story doesn't add much beyond what was covered in the original, and the ending will likely leave you feeling you could have skipped the whole thing and gone right to Halo 3 without missing anything worthwhile.
Halo managed to take a relatively small and semi-linear world and make it feel immense, helped by a sprawling landscape traversed with the aid of vehicles like the Warthog. Bungie claimed even more massive environments for Halo 2, but the actual playable area is usually small, and the sometimes painfully linear paths make them feel like a series of differently decorated corridors than a true landscape. Even when you get vehicles, you'll be riding or flying across a prescribed route, much like the seemingly never-ending tunnel that makes up roughly half your time spent on Earth. While some interiors of Halo were narrow and repetitive, Halo 2 takes this to new depths with downright claustrophobic corridors, often repeated to add to the game length. And will someone explain to me why the supposedly invincible Scarab has a huge open area on top? It's like building a tank with a sunroof.
With such a linear design, I shouldn't be getting lost or stuck, but sadly in Halo 2 that was all too often the case. Aside from the rare on-screen indicator pointing towards my next goal, I usually had nothing more than a vague mission objective and garbled vocal instructions to go by. If I wanted puzzle elements I would have played Metroid Prime 2, especially when exploration in Halo 2 is more like playing Russian roulette, surrounded by invisible walls and easy to fall to areas that grant an instant death, even if it's only a ledge slightly below you.
It's hard for a sequel to retain the intensity of the original, since most elements won't be fresh or new, but that doesn't mean the developer should give up on it. Sadly, that's how Halo 2 feels. The attack on the space station plays out like a poor imitation of the first level of the first game, leaving you fighting all too familiar enemies down narrow corridors and other uninteresting areas. The Flood sort of just show up in Halo 2, a stark contrast from the build up of their arrival in the first game. The first level ends with Master Chief taking a bomb-armed freefall towards a Covenant capital ship, which might have made an interesting diversion from the run and gun gameplay if it hadn't been confined to a cut-scene, much like the better moments of the entire campaign.
Instead of trying to make every enemy count, Halo 2 nearly becomes a Serious Sam clone as it throws hordes of enemies after you, muddling these encounters into one grey mass of blasting and regenerating shields. One of the last areas in Halo begins in a locked room with two weapons and four sentinels, where I counted each death with a small sigh of relief. In Halo 2, you'll be locked in an area with an endless stream of sentinels while surrounded by the Flood, repeated several times over. With the two groups fighting each other, does it really matter if you're there at all?