There are some moments (I mean in expansions) when NPCs comment your actions. I'm currently playing Sith Warrior and I wonder:

1. will someone mention what I did to Darth Baras? (I'm mostly light sided, I don't know if I want him imprisoned or killed)

2. if I don't do Ilum storyline or Hutt Cartel expansion, will Servant One on Rishi (or anyone else) mention that I was on Ilum or Makeb? What is related to Hutt expansion btw? (I simply imagined that another character, Agent for example, should do that storyline, will my plan fail? )

3. I heard that in KOTET Acina remembers you from the Seeds of Rage storyline. What if I don't complete those quests?

Are there any others interesting conversations?

I myself haven't quite reached that point in the game, I'll be honest, however if I got my facts right, if you didn't do a quest and NPCs mention it, its assumed you did nothing but dark side choices. Its the same with all the classes, if a quest was important enough to be mentioned and you didn't actually do it, its assumed you did actions based on the factions your in. (Empire classes will do nothing but dark side actions, and republic classes will do nothing but light side actions)

I think that's how they handled the Eternal Empire and Eternal Throne DLCs for what kind of people the other classes were "canonically". Hope this helps!

Class content is not mentioned often, but Darth Baras' fate will eventually come up again.

If you don't do the Seeds of Rage quest, Acina will say "It is an honor to finally speak to you" when you first meet her in Kotet. If you did do the Seeds of Rage quest, she will say something like "It has been a long time since we last spoke."

I don't believe they specifically mention how you dealt with Baras, no. As the last poster alluded to, there is an eventual pay-off much later in the game depending on what you do. I took my SW all the way to Nathema and didn't notice much continuity with class choices aside from flavor text.

I don't believe Ilum has any relevance in the Makeb storyline.

I don't believe Ilum has any relevance in the Makeb storyline.

Your belief isn't required. It is stated at the start of Makeb as plain as day from Darth Marr. recent conflicts (which include Ilum with Malgus' failed bid to become emperor) has sapped strength from the overall naval forces the empire has and the empire is on the verge of facing a major defeat to the republic, hence the need to have an operative secretly arrive at Makeb and find a means at sustaining the empire through isotope-5.

in fact the whole reason for the makeb storyline on the imperial side was because the empire needed to find an alternative means of survival and this meant bluffing the republic and hutts to leave makeb so the empire can setup shop and mine the stuff.

after makeb i remember there were mentions of iso-5 bombings and the only ships capable of having a chance against the eternal fleet were ships powered by iso-5. there are some references of previous events from previous content.