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.