They're OK. I appreciate the variety of species and storylines, for sure.
I wish they would speak Basic, and I wish even more that when they were interacting with my characters, I got to hear my characters' voices and see their facial expressions. "Classic conversations" are one of the worst things to happen to this game.
Overall I am happy with the mix. However, it would be even better if the "kill X enemies" / "do X missions" objectives were focused on planets with hard-to-grind achievements, like Rishi (for missions) or Iokath (for kills). Sometimes they are, but some weeks I'm trying to find a planet where I can make progress on both seasons and remaining achievements at once, and I cannot.
See also my answer below about fitting into a desired play session.
Get to level 100, yes. Finish 100 weeklies, no. Finish the reputation track, I could if I were diligent, but it's too easy to forget.
Basically, the time-limited achievements for pushing yourself very hard, are frustrating. Maybe you could change them to be cumulative, e.g., after I've done 100 GS weeklies (across all seasons) I get the season 1 title; 200 I get the season 2 title; etc.
When I'm playing SWTOR, my favorite activities are operations and advancing the single-player stories. These rarely fit in with Seasons. When there's a "play 15 missions as (some origin story)" objective for classes I haven't yet driven all the way to the end, I'm very excited. I just got to take my Bounty Hunter through the first part of Makeb last week which was great.
However, to be honest, without Seasons I would probably be prioritizing other games,over SWTOR operations and single-player stories. So, good job on the player retention mechanism :)