Retribution makes Chaos Rising look like a waste of money
The depth of Retribution's campaign has much more depth. Now rather than do a mission, receive a reward and move on, you have to choose a reward that unlocks either wargear, abilities for units or whole new units. For example a typical mission will off a purple gun for one of your heroes, shotguns for your scout units or unlock terminators for use.
Talents are back as well. But this go around, no talents go to waste. Talents are simplified in a manner that you don't have to drop a bunch of points into 1 spec to get 1 ability. 1 talent point will unlock 1 talent ability. These talent points also upgrade your various units. Like for example the techmarine hero''s talents can now augment terminator units or dreadnaught units (depending on how you spec).
The only con I can give the single player campaign is you only get 4 heroes. That's not a big deal as in previous titles you would use really only your best 4 and not even bother with the rest. To make up for this, the previously mentioned talent trees customize heroes. One hero can become a devastator type unit or an assault marine type unit. So you build your heroes how you prefer to play versus having a hero pre-designed to fulfill a certain niche role.
Multiplayer is the same as it has always been. Every faction received a new unit and Imperial Guard is now playable. However, THQ dumping games for windows for steamworks is a huge upgrade.
So to recap, this is the best game in the series. It isn't just another expansion. It improves on almost everything. 9.5/10 from me.