I'll talk up Japan a little since alot people here seem to be rooting for Australia.
This will be a midfield battle, both teams have their strong points there. I saw Japan in person when they played the United States a few months ago (result was 3-2 in favor of the USA), and I must say their defending is circumspect. The exception is Alessandro Santos (Alex), who is a very solid left-back who can get forward and attack as well and could very well start on the left wing. I expect him and Emerton to be having some battles on that side of the pitch. I don't know much else about who stands out, as Akira Kaji is injured and I would guess Nakazawa could start at right back and we can see the captain, Tsuneyasu Miyamoto at center-back along with perhaps Yuichi Komano. Also, Japan seems like they could play with three defenders in which case I could say Nakazawa, Miyamoto, and Komano would be up there.
Their midfield is strong and deep with two Nakatas (Koji and Hidetoshi), Nakamura, Inamoto, Shinji Ono, Fukunishi, and Yasuhito Endo. There are a few more midfielders in the squad, but all of these guys are good. Alex can also play on the left-wing.
In attack the man to watch out for would be Naohiro Takahara, who plays for HSV. The ideal partnership for him would be Atsushi Yanagisawa, and he is back in trainingafter a foot injury, but it looks like he will start.
My predicted lineup (3-5-2):
Takaara-Yanagisawa
Alex-Fukunishi-H. Nakata-Inamoto-Ono
Miyamoto-Komano-Nakazawa
Kawaguchi