Source of Tobi's Power?
I've got to be honest, This latest chapter really confused the hell out of me. How is Tobi so powerful that three of the strongest ninja in the world can't lay a single hand on him?
Kakashi is a master of ninjutsu with a huge catalog of copied techniques second to almost nobody (Orochimaru perhaps knows more, but without his hands, can't use them). Gai is a master of taijutsu with almost nobody in the entire manga (except, maybe, the Raikage) at the same level of skill. Naruto is the strongest jinchuuriki and the best (human) master of senjutsu. On top of that they do have Killer Bee there for support, even though he's too wounded to really play a very active role. At his best, Killer Bee is an extremely powerful ninja with taijutsu powers similar or better than the Raikage. Even if these four ninja aren't the absolute best in all cases, they're still pretty damn good.
Meanwhile, who's on the other side of the field? Tobi. Itachi described him as being a "shell of his former self". Maybe Itachi was wrong, and maybe Tobi had pretended to be weak to fool Itachi, but he's a bright guy and we should accept what he says most of the time. Minato defeated Tobi in a relatively short one-on-one battle. Konan almost defeated him, except he sacrificed an eye to cast Izanagi at the last minute. Even Fuu and Torune managed to destroy one of his arms in a relatively short battle, and might have been able to do better if they understood his powers more.
So how is it that Tobi, with everything that we've seen so far, is able to fight three-on-one against three very powerful ninja, with Killer Bee in support? What is the source of his power? In the manga, we've seen only a few different types of power that people can draw on:
1) Natural chakra reserves. There is some fluctuation and some people have more powerful chakra than others, but very few people have the natural chakra reserves necessary to stand up to a jinchuuriki by themselves. Tobi may have large reserves, but Naruto also has naturally powerful chakra AND added power from the Fox. Uchiha, which we assume Tobi is, are also not known to have as much physical energy as the Senjutsu (which we assume Naruto is, or is closely related to). In fact, the Uzumaki are particularly well known for having powerful chakra, more than the Uchiha ever were.
2) Training, which can help to increase chakra reserves and make chakra use more efficient. Tobi has had years and years to train himself, but Gai is a training maniac too, and Naruto has been through some intensive training as well. Considering his age, can Tobi really have that much of an advantage here?
3) Senjutsu chakra, which requires great chakra control and large chakra reserves, plus it either requires some sort of merger to use it or else it's time-limited. There's no evidence that Tobi has done this, and there's no evidence that he's fused with some kind of animal helper to give him an unlimited supply of senjutsu chakra. We've also heard before that you can't collect senjutsu chakra while you're moving, unless Tobi has found some kind of secret to do just that.
4) The will of fire, where ninja are said to get more powerful when they are protecting something they care about it. Tobi clearly doesn't have this, and is very dismissive about it.
5) Revenge, anger and the cycle of hatred, which Sasuke drew on to become something of a powerful avenger in the fight against Danzo. Tobi may have this, but he doesn't seem to be acting in a rage like Sasuke was. Tobi is much more calm and confident than Sasuke was.
6) Medical Manipulations, like those used by Orochimaru and Kabuto, to increase natural chakra reserves and add additional powers. Even if he has some medical enhancements, it's likely not to the same level as Orochimaru or Kabuto have developed, since they weren't allies for long. The only obvious changes he's made to himself are the addition of the Rinnegan eye and some sort of physical connection with Hashirama or Zetsu.
7) Being a jinchuuriki, which Tobi can't be because all the tailed beasts are in the statue now (except Naruto and Bee).
There are three very powerful ninja fighting him, and yet Tobi seems to be more powerful than them combined. How? What is the source of his power? And if he is so powerful, how could Itachi have possibly called him "a shell of his former self?". Was Itachi that wrong, that he couldn't even recognize one of the most powerful warriors in the entire world? Is it really possible that Tobi, by himself, is more powerful than his 6 jinchuuriki, Rinnegan-enabled, Edo Tensei immortal, unlimited chakra, 6-path zombies? Naruto defeated those guys, but Tobi is more powerful than them combined? HOW?