Well, there are a lot of dumb characters.  Diana troy, riker, dr crusher, etc.  deep space nine, enterprise, and voyager had even dumber characters.  This is probably my biggest problem.  I just hated most of the characters, they annoyed me.  Their dialogue was cringe worthy at times.  A lot of them just felt robotic or devoid of personality.  If you dont buy into the characters you are watching and the actors playing them, you arent going to enjoy the show.  This is why jj abrams star trek movies are so much better.  Even spock feels like he has a real personality and the dialogue and characters just click.  

A lot of the stories were very predictable or just plain bad too.  Everytime they did a holodeck episode you just knew it was going to be dumb. I didnt like the week to week approach either.  They werent too bad when they had a focused overarching storyline like the war in the last season of deep space nine, but these shows usually went for the filler episode type.

Why do you consider these characters dumb, please explain.  Same with holodecks, several of those episodes are very well done, "Worst Case Scenario" one of the best imo.  Also DS9 had arcs pretty much all through its life, with season 3 onwards making them far more common.  Voyager had arcs during series 1-2 also, though they went for more individual style later on and settling for the same species in several eps as they reach different regions of space.  Enterprise went from individual to arcing with series 3 onwards.