I grew up with Lego in the 1980s; back then they had zero or few tie-ins with branding other than Shell IIRC. There were also fewer specialized pieces, meaning greater emphasis on kids' using their imagination.
E.g. if you gave a group of kids a "classic" set and another a themed set, with no further instructions, would the first group produce more "creative" designs?
I think the critics of the branded sets undervalue the Piagetian concept of scaffolding, basically that having the Harry Potter superstructure allows kids to riff on a variety of concepts that actually may be more "creative" than the random geometric blocks.
E.g. "What if we built a jail around Voldermort instead of killing him" seems like an equally valuable expression of creativity as "I made a vaguely car-shaped thing, but it has six wheels instead of four and has those little lever bricks instead of a steering wheel."
- Older set has a big baseplate which enables a child to start building a new structure easily
- Older set has well-pronounced architectural elements like arcs and slopes and feels like a solid structure, newer set feels airy and ad-hoc
- Older set has minifigs with a simple paint, newer set has cartoon-like characters
- Older set has a more realistic colour scheme, newer set again feels cartoonish
- Older set has a specially designed backside for playing and putting minifigs