A story that's, in my opinion, superior to the original Homeworld
The biggest change one will notice in the gameplay is the ability of the mothership to move around. After playing Homeworld, I was itching to start immediately without getting over those tedious tutorials first. The absence of a large compass-like reference and marked horizon literally made me lose my bearings. However, these are easily overcome, and once you do it would be hard to go back to the original limitations.
Moving the mothership means a great thing. It changes the field of operations that makes it different from the standard RTS gameplay. The Kuun-lan is designated a "warship" in the end and this is not just a label. Unlike in the previous installment, the Kith Somtaaw mining vessel is not just a rooted base, in the final missions it is actually required to finish the level/game because of its powerful canon.
New changes includes the ability of the acolyte to merge into a small freighter, a multi-beam frigate that is devastating to fighters, a Hive ship which launches drones and the cloaked leech ( a new concept that can destroy/cripple an enemy ship in many ways). Most memorable are the Dreadnaught class and the superacolyte. Enjoy these two vessels while you can because even in Homeworld 2, IMO nothing comes close to the power of these vessels.
Story
This is where the game really shines. If Homeworld is lauded for its story, IMO Cataclysm brings the story closer to the player. You start as a small Kiith, with little or no political influence. This is not just shoved in your throat but in the course of the game, you get to experience this. There is an instance when you Kiith goes to help and is not officially acknowledge in gratitude. There is this crippled ship that you saved who in the end just dismisses you and refuses to help you. The player will feel these discrimination which is a big plus to the story.
Next is the main plot of the game which involves "the Beast". The emotional complexity of fighting ships that devoured your fellow people, and the confrontation with the Bentusi are unforgettable. Even Homeworld and Homeworld 2 lacks this level of interaction with the Bentusi. I don't want to reveal any further but the conversations are "deep" and serve to develop characters even more with each line said.
Once you finish the game, and see Kiith Somtaaw raised into the ranks of the Warrior Kiith, you'll feel pride and satisfaction. Sort of like "redemption" from how you were treated earlier.
Music: The ambience it produces is top notch and is ideally suited for the gameplay.
Graphics: Nothing special in today's standards but definitely an eye candy during its time
Overall
When I think of Homeworld, Cataclysm always comes first into my mind.