Review of the campaign part of the game only. *Spoilers*
In the RPG side of the game, where Raynor often would walk around and chat with his crew, Kerrigan converse too although she does it in a very different manner. Mostly it's talking down to the NPC or establishing chain of command, there are a few lore ones that helps lift the story.
There are a number of NPC's for Kerrigan to talk to but the one that stands out for me is the Evolution pit master Abathur. He's a homerun in terms of playing the role of the mad scientist with his hands together and always being eager to improve, devour and alter all genes around him. His way of speaking in punctuation form is funny and goes to show that this organism goal is to trim off everything that's not needed to attain a goal. He's disgusting that's for sure, but well made. As for the other NPC they aren't that standing out, even more so the Zerg version of the Terran adjutant, Izsha who for a lack of a better word seems annoying.
Kerrigan levels up as reward for completing bonus objectives and main missions, her new abilities can be repicked anytime outside the RTS part of the game, this adds a lot of fun experimentation not having to worry about choosing the right ability. The same goes to unit alterations, Abathur offers to change the core Zerg units whenever you should wish it outside the RTS part. An example could be to have faster running zerglings or faster attack speed. There are 3 alterations for each unit and it make for some fun tactics alteration even more so when attempting brutal difficulty. Abathur will have his Evolution missions opened when the main storyline is progressed beyond certain points. These are small, easy, missions to choose what kind of mutation you want in that type of creature. Example: the zergling can be transformed into a leaper or more zerglings for the same buck. These alterations, strains, cannot be repicked while the campaign is in progress so more caution is advised. However once the campaign is completed the "Master Archives" are unlocked allowing you to play any mission, with any presetup you want. This is a great new way of adopting and inviting replays of the individual campaign mission, and it's meant as an easy way of approach to unlocking all achievements you'd want.
In regards to difficulty the game feels easier than the last, although that's quite possible due to the amount of Starcraft WoL I've played beforehand. However brutal is still a rush, and gives the challenge at least for people on my level. The length feels shorter than WoL but not by a great margin.
There are no choices to make in regards to missions or sides to pick, like the WoL mission in Haven. There aren't nothing like the protoss missions in WoL, however the evolution mini-games are there instead. All in all a great expansion with a great story and gameplay.