First and foremost: If ecchi is well-done, GREAT. If it is the whole effing plot, NOT GREAT.
Story: Bosom. Breasts. Boobs. Tits. Ta-tas. Knock--OH SHUT UP. I'm sure there is more to this show than just THAT.
Seriously though. The near-constant ecchi and nudity detracts from the REAL plot. A young pervert, Hyoudou Issei, is busy trying to simultaneously grope his classmates and look up their skirts when he finds a girl who wants to go out with him. This is suspicious in itself, but Issei is overjoyed at the fact that such a kind, charming girl (with large breasts) wants to be with him. They go on an awkward date and then she... transforms into a fallen angel. Wearing minimal clothing. Whoopdeedoo. She ends up killing Issei, who is all depressed because he never got to feel her breasts. Suddenly, he is turned into a demon by Rias Gremory, a classmate of his. He is immediately drawn to her beautiful face, long red hair, and breasts. And pantsu. And the rest of her body. Her personality? Dafuq is that? I see boobs, not brains.
Plus, they add a bunch of religious stuff, which is a surefire way to say, "I WANT TO MAKE A SMART ANIME BUT DON'T KNOW WHERE TO START!" to the viewer. People! "Death Note" is smart, "Bloody Monday" is smart, "Umineko no Naku Koro ni" is smart! (Though the latter DID add religious symbolism. The lengthy explanations made it work, though.) AND they add that overused "chess game" anthology. Mrph.
Actually, here's the short version:
Yes, I went there.
Animation: Stock "sexy girl" character designs, meet typical harem guy and pretty boy. Not much more to say on that front. Animation is mediocre, with less-than-average action being balanced by the obviously-thought-out "boob jiggle" animation. This is not good. Ah, and the ecchi. Too many nipples detract from the serious tone. Too many bouncing breasts detract from the serious tone. Too many panty shots detract [...] tone. (A note to the girls of "Highschool DxD:" There is an invention out there. It's called the bra. Trying wearing one sometime.)
Sound: Catchy J-pop themes, but you'll forget them soon enough. (I can't say the same for the animation during the themes, though.) The voice acting is fine, but it can't be difficult to act like this, can it be? I have no complaints, but I can't applaud it, either.
Characters: Everyone is some archetype, and there is little development.
Hey! No picture (at least not for now) due to the fact that the boys are practically non-existent, and every single freaking picture is ecchi. Look 'em up yourself.
Hyoudou Issei: Your typical hero. He starts out all wimpy, but soon gets his gigabyte-megapowah-sharingan-bankai-bringitbitches power and kicks major ass. Aside from the fact that this is annoying as hell, he at least isn't a total douche. I mean, he shows some respect for the girls, despite his perverted ways (cough cough MAKOTO cough). But, seriously. I think he sees beauty as being only skin deep. He doesn't care how kind, loving, sweet, intelligent, funny/or witty you are. If you have the right measurements, you're all good.
Rias Gremory: Is she more than a pair of breasts and a butt? No... yes... um... maybe? She's powerful, but I can't shake the feeling that she's just there for the sake of 4chan. (Oh, crap! I just mentioned the unmentionable!) Yes, she DOES have an enviable body. Yes, she DOES have fabulous hair. Yes, she DOES forget her bra. But that doesn't make a good character.
Asia Argento: Nun-meets-whore? Let me explain: We first meet her by getting a nice shot of her panstu. We then learn she is a nun who recently moved to Harem Country. Issei gets his ass handed to him by a priest, and his new disciple, Asia, recognizes him and tries to save him. The priest slashes her clothing and bra (she wears one) and conveniently misses her skin, so we get a nice shot of her nipples. So, our awful priest tries to rape her. (Quick! Somebody guess her cup size!) I'm sorry. Nun... naked and breathing heavily? It's just so... no. Just no.
Himejima Akeno: Someone wanted a sadist so that guys could envision having hardcore S&M sex with this girl. Her breasts are about as large as most swimming pools, giving Rias a run for her money. Unfortunately, sadistic people can be either well-done or not-well done. Sonozaki Shion, from the scary-as-hell anime "Higurashi no Naku Koro ni," is an example of a well-done sadist. Akeno, however, is not. Akeno often Pikachu-shocks people a few times and then steps back and says something mildly-sadistic is a sexy voice. Not even NEAR scary.
Toujou Koneko: This show needs a loli.
Kiba Yuuto: Pretty boy. In case girls happen to watch this show.
Everyone else: GTFO.
Overall: I don't care how high everyone else rates this thing. I think it's another ecchi harem... just, really ecchi. Really really ecchi. Really really REALLY ecchi. And not much else.