I've mostly seen them alter moods and sims will react to them, especially music. They will notice if music they like or dislike is playing. I sent two sims to Selvadorada and I forgot that one of them disliked dancing so she was miserable in the cantina when I tried to get her to dance. When sims with very different likes and dislikes are in a romantic relationship, they get an "opposites attract" sentiment and when they have a lot in common, they get a puzzle piece sentiment. For the one who disliked dancing, I switched her to liking it right there because - she learned to love dancing with her spouse in Selvadorada. Hey, it did add to the experience.
I also agree that it is mainly just detail if you want that. For DHD, it does influence the challenges. The only wonky thing that I have noticed is that sometimes sims will wake up in the middle of the night to have an opinion about some piece of decor in the house. It's probably a slightly annoying bug but it isn't a big deal.
Generally, I'm glad that they added them. They help to define a character more and differentiate different sims. Hey, you might not care that one sim likes jazz and the other likes new age but some players do.
I know they are in discussions about how they might update the basegame to appeal to more players. On my end, I skipped the game for years and only entered with a package of several packs so I never played just the base. There are different things that I've liked about each version of the game including this one. This one is good for sandbox rotations especially if you are mainly a storyteller. There were actually things about TS3 that annoyed me though yes, there are things I miss. Most of all, I'd actually welcome a new Sunset Valley complete with the openness and the swimmable world.
I do think they need to work on the whole area of interests, hobbies and unique behaviors including approaches to romance. I think a lot of people just told them that so maybe we'll see something.