It's a mix of things. I don't think all the 25th parts are outdated, but a lot of parts definitely are. And range of motion is surely better on the newer sculpts. Elbows are a HUGE difference, and the shoulders seem to fit better on the torso.
One thing they seem to do on the newer ones is have smaller waists with slightly too large hip joints, but almost all of them have webgear or a vest that properly hides it and bulks them up.
For me, it's not so much the sculpt work detail, but the proportions for the most part. Stalker at least still looks okay because the camo helps hide some of the inconsistencies, but that Joe greenshirt is all sorts of awkward. That SE torso was way too short, and all the legs/arms that go along with it look strange now.
There may be the same amount of detail in some of the sculpts, but the shapes are better. I don't know how better to explain it, but it really stands out on bare flesh and heads/helmets. Take the Gung Ho torso and arms, and compare it to the newer Quick Kick bare torso and arms. Or 25th Roadblock's arms compared to Retaliation, or even Resolute RB's arms. The form and structure is more defined and looks more human, and less cartoony.
I agree with the bare skin and especially the heads. Newer heads have much sharper detail than the 25ths.