The Dawn of War series has always been a little different from other real-time strategy games. Based on the Warhammer 40K strategy board game, Dawn of War tries to shift the focus away from resource collection and management, and instead focuses the gameplay on your units and of land control. Capturing special resource points gives you requisition, a resource which allows you to build your units and buildings in combination with energy produced from power plants.
Dark Crusade is a hybrid expansion for the series which brings in two new races to the fray and an entirely new kind of single player campaign. Though labeled as an expansion, it doesn't require the original version of the game to play, and will allow you to play the single player campaign with all seven races from Dawn of War. In multiplayer, however, you'll be restricted to the two new races of Dark Crusade unless you've got the previous games.
The single player campaign is probably the most different addition to the series. Instead of the story-driven campaigns of the previous games, in which you would play one battle after another for a particular side of the story, Dark Crusade takes more of a strategic approach to conquest. First, you'll be given a choice from any of the five races from the previous games as well as the Tau, a race who fights for the Greater Good, and the Necron, an undead combination of flesh and steel. With your race selected, you'll begin a campaign of conquest across the planet of Kronus. The planet is divided into sections, and any time you want to take over one, you'll have to fight a battle against the forces stationed there. Your mission is to take down your opponents' main base of operations while making sure your base stays safe.
The new campaign style is pretty interesting, allowing you to manage armies and requisition on a global scale. The ability to control any and all races is a welcome addition as well, but it comes with a price: the story. Because the game is based around non-linear conquest, there's no story for any of the races beyond for an introduction cinematic for each of them that states why they're on the planet Kronus in the first place. It's a big tradeoff.
Another interesting addition to the single player campaign is the ability to upgrade your commander and his honor guard. As you accomplish certain objectives, such as obtaining a 3-to-1 kill ratio or a certain number of conquests, you'll acquire war gear for your commander that you can choose from a list. The war gear gives your commander a new look and boosts his abilities, allowing him to become even more powerful. As you conquer locations, you'll also get the ability to train an honor guard, units that are more powerful than the ordinary troops that start already trained at the beginning of every battle, right next to the commander. These incentives add more strategy to your choices of conquest locations because certain places give you the ability to train particular units.
The biggest problem with this is that there's no real difference between playing as any of the races. Sure, you're able to control another race, but the entire game just feels like a series of skirmishes (save for a few special locations that give you special conditions). Defending the same location over and over again can really get old quickly.
The other changes in Dark Crusade are a lot slighter then just adding new content. For one, the stealth ability is now more of a constant effect, instead of something that has to be triggered. This means that certain units that can detect infiltrated (stealthed) units are much more important, and it means filling the battlefield with high-level units will not work as well anymore. Though that's been changed too: restrictions have been placed on how many of certain units can be produced. This prevents players from simply reaching the highest level of units and building as many of them as they can. Both of these changes means that much more strategy has to be put into what units are produced, and makes the game a lot more fun to play.
Aside from that, the two new races, and a new unit or two for each of the others, there's not too much else to mention new to the game. The audio is still pretty good, and some of the music can really get the blood pumping. Graphics are as good as they've been before, though the campaign map feels a little constrained.
The idea of a hybrid expansion is a good one, so that those who already own the game can expand their multiplayer experiences, while those who don't are still able to enjoy the world of Warhammer 40K. The new single player experience is a big change, but the lack of a good story might irk some people who'd like to see more of the world fleshed out beyond battle after battle. It's the same tried and true Dawn of War game, but with enough new content to have a good time with.