One of the most ambitious expansion packs ever made.
The previous expansion, Winter Assault, was a great addition to the core game which included four races. WA added an additional race, the Imperial Guard, which were every bit as well detailed and fleshed out as the other teams. It also included some serious rebalancing, which fundamentally change the gameplay... some said it was for worse, others said it was better.
Dark Crusade follows the trend of Winter Assault by adding new races, this time two, a new campaign, and some game restructuring and rebalancing. It also includes five new units, one for each existing team. These new units are all fairly well done, but for some (the Harlequin, Daemon Prince, and Grey Knights) one wonders if they were necessary at all. Another (the Heavy Weapon Team) are very useful, but at the same time highly clunky and noticeably glitched. Overall, however, they are good additions and add at least a bit of depth to the existing factions.
The new teams, the Necrons and Tau, are amazing considering not only have two been introduced at once (extremely rare for an RTS of this calibre) but they have much of the same excellent polish and intrigue as the original teams do. The Necrons are soulless, robotic warriors whose goal is to extinguish all life in the galaxy. They move with a convincingly slow, ominous pace and some of their higher level units are quite imposing. The Tau, on he other hand, are a more benevolent race whose goal is to bring unity to that galaxy through an ideal called the "Greater Good". They are, however, more than willing to use force to achieve this unity, and focus wholly on ranged combat whilst eshewing melee.
The visuals have been improved somewhat over the original Dawn of War and Winter Assault, which is quite impressive with high-resolution textures enables. The new animations and designs are all great and work well with the existing ones. Most importantly of all, I would argue, is the fact that the new "risk-esque" campaign map is a success. Moving your force from territory to territory provides a unique strategic element which helps bolster the game, through acquiring new "honor guard" units which follow you around and earning "wargear" to improve your main hero. There are varied objectives added in alongside standard battles, too; storming an enemy's stronghold is quite a treat. The only downside is tha the AI is sometimes too simple or will overwhelm you with an unrealistic number of forces, but on Normal, none of these things were TOO overwhelming (though some were definetly hard... which is a good thing).
Finally, some of the new voice work is noticeably worse than it was previously. Some of the new leaders sound pretty bad, and these new voices have permanently replaced the old (good) ones. The units themselves, however, aren't so bad.
Though the game is somewhat unbalanced with the new factions, and there are a couple of bugs and poor voicework, this is understandable considering the massive amount of new content added. Dark Crusade is somewhat rough, but regardless is incredibly fun and one can easily forgive it's foibles (which will most likely be patched in good time) to partake in the massive amount of meaningful and engaging new content it offers.