TNG marked a new, different Trek. The lines of ships were different, Starfleet was friendlier to aliens, the miniskirts were gone. For the first two seasons, Roddenberry remained a great influence on the show; then, he died, leaving the franchise to be directed by others.
There are new enemies. The Klingon Empire is largely friendly now, and honorable warrior-poet types rather than bitter competitors. The threat: The machinelike Borg, conceived as a threat so inhuman as to be akin to a natural force. The vicious little Ferengi and the hostile Cardassians in late TNG rounded out the measure of things; the Romulans, having been enigmatic and withdrawn, show up again - bolder, more impressive, and as difficult to grasp as always.
The Federation is little larger than before; ships are, if anything, slower. Torpedos are much more extensively seen.