The show kind of has two period. The first is the more logical one: Clark in high school, still not sure of himself, who he is, where he is going, and fighting a destiny he didn’t want (as noted, teen angst). Make it through four years of that and they have enough for syndication. On the flip side was a young Lex Luthor, equally fighting his fate of going down a dark road, trying to find the light. Not exactly by the book, but it wasn’t too bad (helped to watch it with my son, who was about four years younger than the characters were supposed to be). Some good supporting actors in Michael Rosenbaum, Allison Mack, and John Glover, balanced by some annoying ones in Kristen Kreuk and Sam Jones III.
After high school, it seems they wanted to skip over college and put them into more adult situations. The whole Clark-Lana relationship got more and more contrived and overblown, until the actress finally did the fans a favor and quit. The logical end to the series was Lex completing his journey to the dark side (which he did, and the actor recognized, and wanted to move on because of that), while Clark completed his jounery to becoming the guy in a cape (which hasn’t happened). Basically because the CW doesn’t have a whole lot going for it and they seem to be able to at least make money off this series. Basically, it’s burned out its main thematic elements and all but two of the original cast, though, Lois (Erica Durance) was an upgrade over Lana. But it is more of a superhero comic book now than originally.