I've noticed, reading Naruto for the past few years, that Kishimoto loves to create enemies who have weird powers. That's fine, of course, because if everybody used the same attacks, the series would be boring as hell. The problem, however, is that Kishimoto does not try at all to explain how these odd creatures came to be.
This isn't really a big problem throughout the first half of the series. Zabuza is just strong and uses strong techniques, and Haku has a Bloodline Limit. (The Bloodline Limit thing is kind of a cop-out, but I'd say it's a necessary device, so characters can have different attacks, like Bleach's use of Zanpakuto.) With the Sand attack arc, it's the same - characters either have strong techniques or a Bloodline Limit. However, when the Rescue Sasuke arc starts, we see a bit of the problem. Orochimaru's henchmen are all weird, with no explanation whatsoever concerning why they are the way they are. Kidomaru, for instance, has six arms and can spin spider webs. That's kind of weird, isn't it? What kind of Bloodline Limit is that? Then, there's Sakon and Ukon, who are two people but one person? How does that even make sense? Now, one could argue that these oddities are due to Orochimaru's experiments or whatever, but that seems kind of weak to me. Still, I guess it works.
The members of Akatsuki, however, make no sense whatsoever and cannot be explained at all, it seems. Zetsu is a venus flytrap, for God's sake! And two people at the same time! What? Then, we have Kisame who has blue skin. Sure, he's from a village connected to water, but what kind of person is born with blue skin, even if the village is water-based? The list goes on: Konan can turn into paper, Deidara has mouths in his hands (and apparently cut out his heart to put a giant mouth there), Hidan couldn't die, and Kakuzu could steal people's hearts and "die" over and over again. None of it is explained at all; no reason is given, not even a completely infeasible one.
Maybe that's why Kishimoto hasn't tried to explain these weird people, because none of it would make any sense. Still, for a mangaka who tries to make his manga seem intelligent and steeped in realism (by having characters actually plan out attacks during battle and such) to throw all reason to the wind and not even say, "It's a Bloodline Limit," it seems odd, doesn't it?