I don't think Yamato is that powerful per se. I think it's something to do with his bloodline, Wood element is probably the thing that can go against Bijuu's (?). Everybody says that Yamato was the right man for the job because he inherited the First's abilities.

Makes me think whether the plot runs any deeper. Probably the First was doing research on these monsters or something... Probably thats why he could stop the war with the Second and create the Konoha village.

And Sai meeting up with Orochimaru? Hmm.. Interesting. I wouldn't be surprised if Roots used to admire Orochimaru and probably rooted 'tee hee' for Oro becoming the Hokage. They are extreme in their methods, well, so is Orochimaru. Probably have affiliations and might want to retake Konoha.