A new faction was included, the Imperial Guard, previously available in certain single player missions of Dawn of War. The military unit featured in Winter Assault is graphically similar to the Cadian Shock Troops.
The Imperial Guard specializes in defense, and therefore has the strongest defensive structures of all the factions in the game. Their infantry tends to be of lower quality than that of their opponents in terms of weapons, armor, and morale, but the Imperial Guard makes up for this with a range of powerful tanks, such as the Baneblade and Leman Russ, as well as the walking Sentinels, which, until the Dark Crusade expansion, is the only vehicle that can un-capture strategic points.
There are two new single player campaigns, both of which involve each faction attempting to recover a lost Imperator Class Imperial Titan. Three of the races from the original Dawn of War (Orks, Eldar and Chaos) are available in addition to the new Imperial Guard, with each race trying to find and gain control of the Titan for their own reasons. Although the Titan as a whole is not controllable, its weapon systems can be used to assist in the destruction of the Necrons, who make a cameo in the final mission for each faction.
Many of the units available in Dawn of War were simplified, reduced or limited for the Winter Assault expansion. For example, Space Marine Predator tanks were limited to carrying anti-tank weapons while Chaos Predators were limited to anti-personnel weapons. In the original Dawn of War game both sides could upgrade their tanks from anti-personnel to anti-tank (Destructor pattern and Annihilator pattern respectively). Likewise the Land Raider was limited to one per player.
Limitations were also applied to the Orks, who lost most of their upgrade choices and became limited in their weapon choices. The Eldar had their "hard counters" removed, meaning that specialized units such as Dark Reapers were no longer as prominently effective against heavy infantry. Some units, including most of the Ork army, lost the ability to use grenades. The Chaos Space Marines also lost the ability to upgrade to any heavy weapons other than the heavy bolter.