Soulstorm is different in many ways compared to its predecesors some of which are good and some of which are bad.
Soulstorm has 2 major additions which are:
Races:
The two new races are the Dark Eldar and the Sisters of Battle.
Sisters of Battle - are a sect of all women fighters from the Space Marines. In many ways this is not very interesting because we already have the Space marines and the Imperial Guards and the Sisters of Battle are somewhere in the middle. They have similar units to the Space Marines but are more occupied of cleansing 'impurities' and there soldiers reflect this.
Dark Eldar - are a break away group from the original Eldar group. The Dark Eldar are very cruel and normally take slaves after world raids to torture and experiment with. Their soldiers while quite weak have very strong weapons and strength which balances out. They too aren't that original as there is already the Eldar race which has been there since the start.
Aerial Units:
Every race now has a flying unit with the exception of the Necrons (damn). Many people think that this is a good thing I however think that you'd get more fun out of an update like a toad on a unicycle (picture it!.....isn't it awesome?). I hate the release of airial units because it means you have to look after your base more which subtracts from your attacking strength.
All in all Soulstorm gains new ground with the new races but looses some with the addition of aerial units. Even after the set back of flying units I think it's the best one in the series to date.