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.