Originally Posted by Eldan
To expand a bit:
Winter Assault adds the Imperial guard to the mix. Their thing is that their units can be moved inside buildings for better defence, and that they have tunnels connecting their buildings. Campaign is Imperial Guard.
I forgot what was special about the Tau. There's one campaign for each race, but they are all pretty similar, in that they are all thrown on an overworld map filled with standard missions to take territories.
Soulstorm I haven't played, but it's apparently quite bad. It adds air support for all races, Dark Eldar and Sisters.
Correction: Winter Assault has two campaigns- a Guard/Eldar campaign and an Ork/Chaos campaign, where you alternate between the two (temporarily) allied races, and then you pick which one you want to finish the campaign with for the last level.
Tau don't really have a GIMMICK per se, beyond lackluster melee and terrific ranged firepower.
Soulstorm's campaign is essentially exactly like Dark Crusade, but with a bigger world map and no interesting missions other than the strongholds (Dark Crusade had variety of special locations that'd give special abilities, and then the battles for these locations were unique. For example, the Mark of Khorne location let you attack twice per turn, but to win the battle for it, you had to be the first player to amass two thousand kills.) The strongholds were still fun though.
That said, Soulstorm adds additional units and UI improvements over Dark Crusade that make it definitely worth picking up if you've got friends to do multiplayer with.