Isuca's the story of an average high school boy that gets hired as an assistant/housekeeper by a girl in his high school. Girl turns out to be an exorcist that hunts down spirits (basically youkai...?), and the guy happens to have a magic power that lets him "control" any spirit/person by knowing their "true name," which happens to involve some form of kissing.
So basically, it's an excuse for the protagonist to kiss or make out with any of the female humans and spirits that'll be part of his harem. It could be argued that Sekirei has a similar concept, yet from what I remembered, I'm pretty sure that it heavily relied on mutual feelings between a sekirei and her master, especially when they'd form a contract or unleash their ultimate powers.
Tora Kiss - A School Odyssey did something kind of similar as well, involving battles that absolutely had to have the two competitors of a battle kiss each other on the lips no matter what within 24 hours, as a way to supposedly "share their stats." As ridiculously convoluted as that idea was (as all the straight boys don't want to fight each other for obvious reasons...), I at least think it could've been used pretty comedically, especially for Isuca: Imagine the protagonist needing to kiss some incredibly hideous spirit/youkai as the ONLY way for them to win the fight and live, and of course, he'd try to stall by making up tons of excuses on the spot.
"But there HAS to be another way!!!"
"You'd rather have us all dead, instead of just kissing that thing once?"
"Uhhhh... Yeah..."
I don't see how it would be "too goofy" to have something like that in the series, especially as it continues to present the usual checklist of ecchi clichés found in every chapter to fill out its quota: The girls' clothes ripping off during battles, so many revealing panty shots, exposing nipples, the girls falling onto the protagonist to create perverse positions, etc. etc. etc.
I don't have an issue with ecchi itself, but for a series like this with very average artwork, not much humor, boring "romantic development," and an uninteresting narrative without much of an overall direction for where it's meant to head, it goes to show that not even the ecchi would be worth reading it.
Omamori Himari finally caught up to its ending a few months back, and I found it doing a better job when it came to its supernatural action and (still pretty clichéd) ecchi/romance. While u can read that as a completed series, a currently ongoing one that also does a better job is Youkai Shoujo - Monsuga. Another similar premise that has its own spirit/youkai-hunting, center-to-a-growing-harem protagonist like Isuca + Himari, and although its romance is pretty weak, the comedy, action and art are all pretty damn good in it, and the fan service can be intentionally pretty funny at times as well.
[Sidenote:] The two main girls of Isuca remind me somewhat of Asuka and Rei from Neon Genesis Evil-Gelatin (Pan Pizza [RebelTaxi on youtube], I must always call it that cause of u...). Never watched the show, but I still feel like they bear a close resemblance...
Evil-Gelatin
Not so great example from Isuca or [url]http://www.mangahere.co/manga/isuca/v04/c039/2.html[/url]