(Warning: This is for a joke)
The Grandaddy of all shitty harem anime.
Issei acts less like a normal anime protagonist and more of a hentai protagonist.
This is soft core hentai, if you want to draw big boobs and girls getting fucked then just make a hentai.
Bro why are all the girls eyes somehow bigger than Uzaki Chans tits?
This truly has started the trend of a shitty protagonist that has no thought put in and all the girls are made to be more interesting than him.
The literal only good thing are the dragon transformations (which I'm not gonna like look actually cool, especially Juggernaut Overdrive).
If one of your main draws to your show is fanservice, then why on Gods Green Earth would you censor it?!
How does Senran Kagura, the Omnipotent overlord of fanservice, have a better fanbase than Dxd?
The fandom somehow follows the crap trend of making "Issei betrayed/overpowered" fanfics. Like we haven't seen enough of those from fucking every fandom, ESPECIALLY NARUTO MY GOD.
Finally, the girls aren't even that attractive. Compared to other anime or hell, video game anime, they're mostly B tier. Hell, I'll give some examples!
Sonoshee from Redline
Baiken from Guilty Gear
Michelle Davis from TerraFormars
And literally most of Bleach's female cast