Shido Itsuka was born in a world where widespread catastrophes of varying size called spacequakes can happen anytime and anywhere, razing everything in the vicinity. When his city's alarm forewarns one impending, the teenager races to his sister Kotori in an attempt to rescue her. But upon his arrival, the spacequake transpires in that very spot, and standing in the middle of the destruction is a beautiful sword-wielding girl. Soon after, he learns she is one of many beings called "Spirits" who cause the disasters, and charming those like her is the solution to preventing them. Effective immediately, Shido must date Spirits and make them fall in love with him, because if he cannot, mankind's destruction may very well be inevitable.
Self-proclaimed, proud pervert Ryousuke loves women – to him, their butts and breasts are a 'treasure box' of joy for all men. So when the boy spots a fiery-haired girl standing in the rain one day, he invites her to his house – but he didn't expect to be stabbed with a magical sword in return! The girl, Lisara, is actually a high-level shinigami who must absorb the spiritual powers of others to survive in the human world. Until she can find a genius with enormous expendable energy, she's decided to form a contract with Ryousuke, but unfortunately for the busty babe, the horny teen's mojo is fueled only through his constant perversion. While looking for Lisara's true prize, the two must deal with other well-endowed shinigami and other foes alike, all while trying to maintain a 'typical' school life together.
Supernatural girl appearing from out of nowhere? A male lead who is barely able to do more than run from the supernatural being before him? Both anime start out about the same however quickly take different turns. While Date a Live truly uses it's harem element. ga Dekinai actually only has it as a side element. This means that there are two things very different about these two anime. ga Dekinai is a power-fantasy while Date a Live is a Harem-fantasy.
All the perverted Issei wants is to squeeze bountiful boobs all day long, so he's thrilled to join Kuou Academy, a once all-female high school filled with gorgeous girls. But little did the boy know that after finally getting a girlfriend, she'd turn out to be a fallen angel who'd drive a lance through his heart, killing him on the spot! Luckily, the beautiful and busty Rias, his popular classmate who's actually a demon, revives him under one condition: that he'll become her servant. Now, Issei finds himself in the middle of a fierce chess game between demons, fallen angels and exorcists, and the fates of his new friends lie in the balance!
Both series feature supernatural girls forming a strange harem around a central male character. While the type of supernatural creatures differ between series both share similar character tropes and body types. Both series also feature a grand overarching conflict. Date a Live is more straightforward and more by the tropes while High School DxD does try to at least stir things up a little. If you have seen many Harem anime Date a Live holds few surprises for you, if that turns you off; don't watch it.
If you call the supernatural magic then both series have a guy surrounded by gals infused with magic with maybe a normal gal or two. Maken-Ki! is the more senselessly perverted of the two and is stacked with filler but Date A Live has it's fair share of fanservice, and I don't just mean the stylish, flashy outfits because all harems need shower scenes. It's a rule.
Both are school-based comedies and are regulated such that their respective worlds become accustomed to their respective situations: Space babes for Date A Live and magical powers and crap for Maken-Ki!. You really shouldn't expect much from either one but both provide some low brain power fun and action to compliment said fanservice.
Fifteen-year-old Tsukune Aono is average in every way; he has no hobbies, less-than-stellar grades and he's even flunked his high school entrance exam. His parents have managed to enroll him in a school, but it's no ordinary school! Tsukune will be attending the Youkai Academy, a place where monsters learn how to co-exist with humans! As the werewolves, vampires, witches and other monsters are required to retain their human forms, Tsukune - the lone human - is able to avoid their bloodthirsty gazes. However, he quickly attracts the attention of a number of devilish beauties including the vampire Moka, a busty succubus and even a young witch! It seems Tsukune's average life isn't so average anymore!
A male lead surrounded by powerful supernatural creatures who so happen to have an attractive female figure. Actually the female figure is probably a main character in it's own right in both anime as it get's a lot of focus. Both series sport a token loli-type girl while the rest are considerably endowed. In both series the male lead is very much the underdog powerwise but Date a Live gives the male lead some survival tools while Rosario to Vampire's anime does not. The premise in both series revolves partially on not pissing of said powerful girls however that's where the similarities end, both have very different settings beyond what I described above but both rest firmly in the harem genre.