Am I going overboard with these images? But I do think that they do correctly depict what the series is all about. Wait, you didn't think that Nisemonogatari was about the story, did you?
To put things in perspective, I could summarize all of the story in all 11 episodes in a single paragraph, and still not spoil the show for you. This isn't really an exaggeration; quite a number of episodes have nothing to do with the plot of the Karen Bee and Tsukihi Phoenix arc, and those that does are mostly filled with dialogue, Nisio Isin style. Leaning on Bakemonogatari's success, I suppose Shaft realized that the fans don't need the story; they want the beautiful synergy between the animating genius of Shinbo and creative narrative of Nisio Isin.
I've gotta say, if it's the story you're looking for, you're in for a disappointment. The length of the story itself is one thing, but the endings to the two arcs are extremely anti-climatic. And I could also draw a lot of similarities from the arc endings in Bakemonogatari, too. There is actually a single moment where I find our protagonist Koyomi really cool, but other than that, there's not much to offer in this area.
So in place of story, what did we get? If the above pic haven't gave you an answer, it's fan-service. Lots and lots of fan-service. In fact, fan-service is the answer to the question in the post title. The scenes above are just a small bit of what you'll expect in the show, and the amount is outrageous enough to spawn the term "pornomonogatari" (I can't remember where I've heard this term, though).
Depending on your tastes, the fan-service can be quite a treat. Unlike the blatant in-your-face fan-service of Queen's Blade and Qwaser no Seikon (which I met with disgust), Nisemonogatari has been very... clever in their deliverance. A lot of them does this without even stripping the clothes of the females; a style I would like to call "suggestive fan-service"; and the infamous episode 8 is a good example of that. I actually think that Nisemonogatari is a bid for Shinbo to see how much of his creative juice he can work into this "suggestive fan-service", and judging from its reception, there's no doubt it worked.
Unfortunately, like some of the others, I can't hide that I find the show pretty objectionable. It feels that the direction of the show implies that it's totally okay for Koyomi flirting around and groping other girls while actually remaining faithful to his girlfriend. Kanbaru actually has no problem being naked in front of a guy, and Hachikuji actually wants to be groped? Well, at least that's a far cry from the fact that Koyomi actually did kiss both his sisters, groped one of them (without consequences) and (suggested that he) went too far with the other. Not that I don't get the thing
But still, Shaft delivered entertainment that probably only KyoAni's over-budgeted productions can hope to match. I feel like such a sheep for liking a fan-service anime, but you know what?