Alrighty, you've left a detailed post, so here's mine. (warning, evidently, it's pretty long XD)
My playstyle is centered around my main Sim going through a long narrative on being "human" (he's actually a divine being, but doesn't know it), then becoming a special kind of vampire to fight against vampires who feed on innocent human beings and other supernatural threats. My game is very heavily modded, so it includes foods for occults and he'll use that to feed himself. This obviously means I'm quite pro-occult, though I don't let them run around in my game unchecked.
Currently, with my Sim as a "human", I'm doing a "completionist" run where I'll have him finish most Aspirations (except the family ones), most careers, and get all distinguished degrees with honors, and this started from a middle-class household. For the time being, he's in a monogamous relationship with his girlfriend, soon to be wife, and will be for a long time, but I'll eventually have him start forming a harem (wife will be supportive, lol), so in a way, I share your enjoyment of poly couples, though in my case, it's just the one guy with a bunch of women who love him very much. Combine that with the vampire theme and the story is similar to the anime Strike The Blood, which was accidental, but eh, it's fun, lol. I'm also a huge romantic in RL, so I like having my Sim romance numerous female Sims. But that isn't to say I'm poly in real life, it's just fun to do so in the game.
Due to the story and some of the lore I came up with, family play is practically impossible for me, which is more than fine with me since I've never been a fan of it. However, for me, it has nothing to do with my views on kids in real life. I'd actually love to have children someday and I have a cat in RL whom I love very, very much, but kids and pets in the Sims have never been that fun for me. However, I like to create alternate protagonists for the story that will eventually meet the main one and I use that opportunity to explore different play-styles, so I may give family play a shot once more if it fits into the story.
And because this is a story, I take screen-shots. A LOT of screen-shots. I currently have almost 7,000 screen-shots and I will easily double that number. I use tons of poses in my game and occasionally hunt for more. With the poses, I enjoy writing out dialogue for the scenes and I also love writing out biographies, personality profiles, etc for the main characters since as a kid, I really liked reading that kind of thing for fictional characters.
Aging is permanently turned off in my game and death only occurs mostly when I want it to or there's a chance it can and it makes sense within the story. I have my Sims maintain their needs for the immersion, but said needs can be filled up and be made less intrusive due to more details I came up with. Another reason I like maintaining my Sims is because I enjoy watching them cook.
For worlds, I like to give the game a sense of geography, so I do not let my Sims travel to other worlds if they feel like they'd be too far away from the others. As an example, if my Sim is living in Britechester, he is not going to go to Sulani unless it's in the context of him moving or going on vacation. He can't just pop into Sulani, then pop out like the game lets you do.
I do not play premades and only use them for background scenery or red-shirt purposes. I also don't download Sims for more characters, I like to create every Sim I use. So when I'm not interested in playing the story, but I still want to play TS4, I like to go in CAS and create a Sim. (heh) I base my Sims off celebrities (sometimes), characters I've seen in media, or just an idea I have. Because of this, I don't create Sims based on people I've met and personally know and I have zero interest making a Sim that looks like me.
I completely ignore holidays. That's actually something similar to what I do in RL. I don't really celebrate them, besides using them as an excuse to go see a movie or eat out.
While I wish I was, I'm not much of a builder. I'd love to be able to create sets I'd need for scenes, but a lot of builders out there have the very bad habit of making their homes not playable, so it's very annoying when I find an awesome-looking home lot only for the lot to confuse my Sims. :neutral:
And finally, because I put a lot of time into my Sims, even exploring TV Tropes just to see what other traits and backstories I can give them, I get extremely attached to my Sims. I have backups for them in-game and on a separate drive.