I play exclusevly vanilla. No mods and no cc. The former because I don't have all the expansion/game/stuff packs and I really don't want to try to dig up older version of a mod that might work with my game. Especially as I am not buying the add-ons in the order they came out.
The second, has the same reason as above. I don't know how safe it is to have a Custom Content if I don't have all the packs. And I am so new to the game (not even a whole month), returning after playing the Sims 3 (which was a LOOOOOONG TIME AGO).
Which means I haven't researched how CC works in Sims 4 and what I have to look out for so I don't destroy my saved games. But to be honest, the main reason is the second reason, which is directly tied into my gameplay style.
I prefer creating Sims, their personality and focusing on their stories. And the Sims 4 gives me an enough variety that I don't really miss CC much. Which is a great change from my past. When I played Sims 1 and 2 I had a crock full of mods and CCs installed but when I got Sims 3 it was the first time I felt like I didn't have to. And it was very liberating.
With the color swatches and the variety of furnitures and other stuff I was finally content. All I did add, mostly, into Sims 3 was the super mod (don't remember its name) that gave me more control over the whole neighbourhood and fixed bunch of other stuff as well as giving me some customization and more importantly. Control. But then I was playing the game from beginning and adding the addons as they came along. Not like now.
Although I do have to admit that more color swatches for some of the objects would be welcome in Sims 4. And I will probably have some CC in my game soon (IF I can) because I would like to create an ultra modern house so new furnitures and even build objects would be nice. I would specially download some of the lots in the gallery. I see a great houses in the gallery that was designed for that empty neighbourhood I haven't even touched it yet, except to put down one of Maxis provided Spa.
But honestly, Sims 3 and now Sims 4 has given me a better chance to just focus on my preferred gameplay style (I am not much of a builder or a decorator) without feeling like I need something.
And as for mods...I am not sure I will add any but I always keep my eyes open and if I like something I will probably use it. AFTER I have purchased all the add-ons which might take a long time because I am only buying what I like and then adding the rest later. Maybe even over a period of two years.