For an Ecchi harem, it has a very interesting story and that's one of the primary reasons I gave it a high grade in this area, it doesn't stack up very well compared to other genres but for what it is, it is extremely entertaining. It has fun with itself and the author seems to be having a lot of fun writing it. They've setup an interesting shounen-style premise with an interesting cast of characters and backgrounds that play off of each other nicely.
Just Issei in alpha mode
There are entire worlds left unexplored in the first season and if you've read the light novels you'd know there's plenty of things left in this universe. The world is massive, the characters are endless and I don't think there's ever a moment when I was bored. One of the best things about DxD is it's ability to completely lose it's mind, as seen in one of the first episodes where a devil is depicted as shooting lasers out of her funbags. The level of comedy in this show is pretty great too and there's plenty to laugh at both now, and in the future when another season arrives.
All that said, the level of fanservice is infinity level so those who are annoyed by it, you'll just have to stay away. It's rampant, there's bare-chested nudity and a lot of it. This doesn't bug me much at all and I've honestly ignored the whole thing and genuinely enjoy the story.
Why can't I hold all this plot?
The animation quality is pretty good, better than other of it's genre but sadly it doesn't stack up quite as well against other big name titles. I am convinced it has some of the best animation the ecchi/harem/comedy department will ever see but it doesn't stack up so well against it's epic shounen adventure counterparts.
I have no problems with this
The battle scenes are fairly well animated but it seems like they've saved some of the budget there to expand on uhm, other fronts if you catch my drift. Many of the ecchi scenes are animated to professional quality while the battle scenes left more to be desired. Don't get me wrong, the battle scenes aren't bad but the fact that you can clearly see more attention to detail in the character's chest areas compared to critical battles is something that should be pointed out.
Just your friendly neighborhood Megazord
The characters are voiced well, nothing seems particularly out of place; The OP for DxD is sort of generic but the music isn't that bad. The ED I'm convinced is one of the best this season (and not just for the epic fanservice), the song is incredibly catchy and fits the series well in both the fun and lyrics department. Not much to say about the sound really, everything fits like it should but there's nothing exceptionally spectacular about it.
What's that? GIFs can't make sounds you say?
The characters are a big part of the story and I've already voiced my opinion on them but I'll try to get a little more specific. Kiba, probably the weakest character of the bunch is Issei's rival yet is one of the few of the group who treats Issei with any amount of respect - not much to say about him, his development would be in the second season (Light novels 3-4) but as it stands he's a pretty weak and undeveloped character.
Though popular with the ladies outside of the club, within the club Kiba is friendzone level 99
Akeno is an S&M freak who likes torturing enemies and of course, is sexy. Koneko is a stoic cat lady who constantly calls Issei out for being a gigantic pervert.
Koneko declares all perverts must die, my life is in serious danger.
Ashia is the resident pure maiden who's being influenced by Rias' perverted devil ways, she lusts for Issei and yet doesn't know enough about relations between man and woman to do much about it. Rias is burdened by her family name and is in an arranged marriage situation, falls in love with our resident pervert when he saves her from that fate.
You'll never take my red headed large breasted woman
Then there's Issei, our main character. He's a pervert, he ends up in funny situations and rubs his sexual exploits in his friends' faces. He wants to touch any and all sexual parts of the opposite sex. He's awesome in that he ends up getting a harem which he's totally unaware of because he doesn't believe women can be genuinely interested in him, when they all eventually fall victim to his pure hearted pervert ways. This all sounds pretty bland and natural for the genre, but they all come together quite beautifully (no pun intended) and it ends up being a laugh riot. There's no typical harem cockblocking or shenanigans here. The enemies are generally all generic except the bosses, but are interesting enough to make you wonder.
Our resident douchebag, Raiser
Let's be honest, DxD isn't going to be winning any awards but it's entertaining as all hell. It's completely insane, shameless and interesting. I've not had this much fun watching an anime in a while. The fun factor is in it's ability to tell an interesting story and it's comedy, there's also some mighty powerful fanservice for those interested in that. (yes, they have bare uncensored breasts, let's shoehorn that in right now) You'll have a good time watching this anime if you can judge the anime based on it's merits in this genre over the level of fan service. Yes, if you hate fan service, you will be annoyed to the maximum extent but otherwise, it's really very fun. I suggest picking it up and waiting for a second season, it's been ranked pretty high in the preorders and sales department and it's sure to have another season (or two, hopefully). It's not the best anime this season but it's certainly not even close to the worst. Give it a shot.