This will be a question, but also somewhat a review and appreciation.
I love DxD. It's one of the few harem anime I have rated onlyMyanimelist. I have seen many people describing it as the greatest harem-comedy-echhi anime of all time. I don't know about greatest, but it sure as hell is popular. And one of my personal favourite. Got 3 seasons and hopefully 4th season coming next year.
The good thing about DxD is, it's not ashamed of itself, and doesn't try different stuff often. The show has remained consistent and faithful throughout its course. It has succesfully built a great cast of likable female characters (no annoying and abusive Tsundere, despite some cockblocking, will get to it below), the MC is a pervert/idiot/brave combo which hasn't changed, the plot has somewhat progressed, all characters have decent background. Despite the crazy comedy and ecchi routine, it still has a progessing plot with potential. But there's no rush to build the plot while sacrificing the key elements (ecchi-comedy-harem). I like almost everything about the show, the blend of comedy-ecchi and harem is near perfect.
Some might say the cockblocking (Asia cockblocking Rias, Rias cockblocking Akeno) might get annoying, but a shonen harem-comedy anime MC never gets laid. Issei's despite being pervert, still refrains from actual sex acts because of his backstory (got betrayed by Raynare early on), preventing him from acting on impulse. DxD despite the bare boob shots, is still light hearted show, not soap opera or hentai. So I have come to accept the annoying cockblock. It keeps the show alive.
I have seen many other anime failing in what they target (Date a Live 2nd season, Trinity Seven second half). DxD has got me hooked for 3 seasons. If anybody hates ecchi just because it's ecchi, I have nothing to say about that. But DxD is success in modern anime in my opinion. Consistency is a key to success.
Why do you think DxD is popular?