Supernatural fights, some comedy, and a relatively weak but fun plot was just enough to keep me watching to the end, but damn, the ecchi is strong in this one. Spoilers ahead.
High School DxD has a supernatural theme, where the main character, high school student Issei Hyodo, is killed by a fallen angel and is brought back by a demon princess and buxom redhead, Rias Gremory, as her “pawn.” High class demon clans are organized in part as chess pieces, with pawns being the only type that can claim the abilities and powers of the other pieces, except for the king.
Over the course of the three seasons, Issei trains hard, gets stronger, and fights supernatural battles on behalf of Rias, his “king” and president of the school’s Occult Research Club. The student council president is another demon princess, with her clan being associated with the student council, and a friendly rival to Rias. We learn that there has been millennia of battles between the angels, fallen angels, and demons, and that the last battle killed not only most of the demon kings (including Lucifer, a title given to the most powerful demon king) but also God and most of the leaders of the fallen angels, weakening the Christian mythological system significantly and giving other systems (like the Norse gods) an opportunity to grow and influence the world again.
High School DxD has the usual shonen tropes – a perverted high school boy as the main character, a harem of girls around him who are all very attractive and want to kiss/sleep with/have sex with/et al him, that one girl who thinks the main character is a pervert (but who ends up falling for the main character eventually). But this anime has way more in the ecchi department than most shonen, to the point that the ecchi elements distract from the actual stories.
As a 43 year old man, I realize that I am not the target demographic for High School DxD. Were I a hormonal teenager again, I’d probably be way more interested in the many, many, many censored breasts, shower scenes, panty shots, and so on than I am at the moment. I mean, 20 or 30 years ago I’d have been whining that this anime was over-censored, rather than complaining that the story takes a back seat to the ecchi elements. I mean, really, Issei’s first custom attack is “Dress Break,” where he magically shreds the clothing of any girl he touches. But these days? If I want to watch an anime that’s one intercourse scene shy of hentai, I’d rather just dive right into actual hentai instead.
The first 12-episode season is the least ecchi of the bunch, and the third season is the most. High School DxD had just enough story and a sufficiently interesting setting to keep me interested through all three seasons. But I would have much preferred it to be more story, less horny teenager.