Love Hina, the series pictured in the thumbnail here is basically the "peak" point for the harem genre, and in a way the entire industry never recovered from it.

Nearly every insufferable trait you can think of from anime was popularized to its overuse from Love Hina. A harem that just pops into existence with one of each cliche checklist girl possible, the most inoffensive MC who is so bland that they all fall for him because he is "nice" and he has no other personality to speak of, a winning chick who is so obviously the winner from the start it removes all stakes from the series, nearly all the gags happen because "guy exists, main girl overreacts, jumps to conclusions, beats him," the MC suffers continuous violence and abuse to a level it ceases to be funny for little justification but any time he stands up for himself he is the bad guy.

I could go on but you get the point. All that stuff that seems to old and cliche now became the "mainstream norm" after Love Hina's popularity. It existed before, but this codified its exact dynamic and it has since then been a plague on the entire industry. Even he clearly saw it was a problem because his next, and arguably more popular, series Negima kept all those dynamics but improved on them dramatically.