The two main problems are, that the show has actually not a real story, nor does it have real characters, as the story is more or less just a couple of underdeveloped mini stories, all centering around a one dimensional villain wanting to rule over the world, put next to each other, while the characters are bland stereotypes - apart from two and a half exeptions.
I mean, the entire "story" (if you even want to call it this) is pretty much: "We get in the digital world. Some villains do appear. We need to fight. Having some sort of revelation makes the Digimon stronger. Hurray, Deus Ex Machina saves the day." The episodes are not only episodic, but highly repetitive - both filler episodes and "story" episodes, since it is mostly the same and even half of the animation is completely recycelt. The series is at its worst in those episodes, where a new evolution is introduced. Those episodes are ALWAYS the same: Character comes up with a new overly dramatized problem. Villian shows up. A lot of shouting. Character realizes that his or her problem is actually not that much of a problem, but just him/her looking at it all wrong. New evolution is unlocked. New Digimon saves the day. Hurray.
And yes, the characters are just horrible stereotypes with mostly not one single bit of originality to it. They just feel like stereotypes. Not like characters. Not likeable. Just stereotypes. Yamato is the whiney, egocentric "rival" to the hero. Sora is "the chick". Koushiro is "the socially awkward intelligent kid". Jyou is the equally socially awkward hard working kid. Takeru is the "little brother". And Hikari simply is a walking, talking plotdevice - not even a stereotype. Just a plotdevice.
Only two exceptions would be Taichi, who is a classy hero, but at least has some decent character development, that happens over more then two episodes, and Mimi, who really is a surprisingly well written character.
The animation is horrible, even by the standards of 1999. A lot of looping. A lot recycles. A lot of still shots. Nothing more to say about it.
Only the soundtrack is quite nice, but cannot make up for the series overall problems when it comes to story and characters.
If this series would have not been the first Digimon series to air, I am quite sure, it would be well hated within the fandom.
