I think part of it is the fact that, in TOS, technology was often the problem. The crew succeeded despite technology, often having to overcome something gone awry to win the day.
TOS was also about exploring on the edge. There was always something new, undiscovered over the horizon, where Starfleet didn't get an answer back on how to proceed until the episode was over (if they could contact Starfleet Command at all).
In TNG, every episode seemed to have a technobabble solution to every problem, followed by moralizing how the Federation/crew was better than the bumpy-head-of-the-week. TNG Trek was "established." It was the late-20th Century Navy, with the rigid following of procedures and established precedent (where Kirk & co. sometimes had to make it up as they went along...even in GO1 situations).
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz