So at the time, the first season didn't seem so bad. It was probably on par with what most people of the time expected. In hindsight ST:TNG may seem like "a decent show with a bad start", but at the time it seemed like "a pretty decent show that just kept getting better". ST:TNG is part of what set the modern bar, and it's a lot of what set the modern bar if you kind of lump it in with Deep Space Nine.
By modern standards it is weak because the tech of the time didn't support season-long arcs because there was very little rewatchability, but if you try to delve into science fiction prior to ST:TNG, be prepared to... make concessions.
Many shows are like movies, where stopping between two episodes feels like you're stopping a movie right at the most intense spot.
You either binge watch the whole show, or rage-quite at 3am never to watch another episode.
DS9 is my favorite though, partially because of the story arc. It was also done quite brilliantly, with the story arc always being present, but not really solidifying into something substantial right away. This along with Babylon 5 were amazing. Prior to these shows, the only television sci-fi I had ever seen with a story arc was Robotech. Battlestar Galatica (original) had something of an arc, but was more like TNG.
- ones with a season-long arc. This gives the authors time and space to develop the characters, and reach a satisfying conclusion. But these often suffer from pacing issues, because mid-season episodes become filler.
- ones with episodic stories and with a season-long arc (a problem or a villain that keeps popping up). This helps with pacing, but still lets authors build up momentum and develop characters and stories. These often suffer from underdeveloped arcs though and unresolved season cliffhangers hinting at "that arc we developed, it's so much bigger than you thought".
I assume that's been less the case for the newer Star Trek universe series. I don't actually know the numbers but the TV landscape is too different to really compare anyway.
Added: As someone who remembers watching TNG when it first came out, my recollection is that the reaction to "Encounter at Farpoint" (S1E1) was along the lines of a bit disappointing but certainly has promise. And it's new Star Trek on TV!