Wouldn't say huge focus of the pack, since you have to focus more on helping them learn how to roll over, sit up crawl and etc, since base game they all know how to do that.
Been playing for a good while, experimenting a bit on families with dynamics along with married couples and found they are such a welcoming addition to the game, especially the social compatibility with conversations and sim characteristics. Definitely makes things more interesting as conversations between sims won't always go well, especially when you watch townies (took a couple of hours when I first got the game to go through played sims and townies to add likes and dislikes to them. They tend to get a randomized one.) Saw townies getting more annoyed with one another if they aren't compatible.
The milestones act like a memory system, but I find it to be a bit more than S2 memories. Sims will remember certain milestones based on where or around other sims it's happened around. Sim had fire on home lot so they will remember it now and again and get tense from it and it won't tell you for how long either it'll stay. (FYI she caused another fire due to the tense memory when she went to cook. :lol: )
Sims being able to get new traits or swapped out based on how they interact is a welcoming change feels a lot more organic like the things they do matter.
Then sims calling you if they can stay over a few days be it friends or family whatever the reason they gives can add to story building or you can invited them over.
I find it refreshing sims can just get burnt out from doing the same things, especially when skill building or working. Less chance to be spamming things with them, as their skills will slow down a lot making things much harder. Workplace rival's for them as well. Adult sims getting midlife crisis (Mortimer Goth got one when I was play testing and had a hunger for success.
I like playing with Elders before, but now they have a few more interactions with other sims. Especially with their families and grandchildren, plus I find the powerwalking good for them since they can workout without being brough to the brink of death.
For me, I find it adds a lot to a sims daily lifestyle and effects it making it great for long term gameplay.