..are the main characters. Just think about it. The friendship/rivalry is just like Goku and Vegetas'. This makes them obvious main characters. The story revolves around them. Narutos' dream is to become a Hokage, and Sasukes' dream is to kill his brother, Itachi. Therefore, Itachi is a main character as well.

They represent the basic character-types. Itachi is a true genius. As far as we know, everything has come easy to him. He became a Chuunin at the age of ten, and ANBU squad leader at the age of only thirteen. There was no challenge he couldn't overcome, so naturally he became arrogant and chose a lonely path. He saw Sasukes' potential, and decided to keep him alive.

As a big brother myself, I understand how he feels. Itachi wants Sasuke to become strong, so he can test his own strength. The bond brothers share is really special (or at least it tend to), and competition is a naturally part of it. It's because of the close connection towards each other, the competition evolves. We can see this in the anime. Itachi care about Sasuke. And Sasuke adores his big brother, which isn't rare in general.

Since Itachi wiped out the Uchiha clan, exept for Sasuke, the world as Sasuke knew it, collapsed. His love towards Itachi turned around to hatred, equal to the idolisation.

Now. Naruto is the opposite of Itachi. His whole life he has constantly tried to keep up with the others, though the outcome was the same every time. He failed. From he joined the academy, he wanted to be recognized from his peers.

Hmm..sorry, I can't keep this up, so I'll make it short. Maybe editing this post later.

This is the important part. Sasuke grew up in the shadow of his brother (of course, his hatred became even stronger because of this). What followed from Itachis' killing, Narutos' rivalry and the fact that Sasuke is Itachis' brother (and therefore a genius), Sasuke got to know both paths. He 'got' some of Narutos' 'never give up'- attitude, as well as the genius-blod which runs trough his veins. This is what makes Sasuke truly special, and potentially can surpass his brother. Itachi saw this very early, that's some of the part why he let Sasuke live.
And I believe Blue stated that 2008 is Sasukes' year, which just futhers this 'theory'.

Of course, since it's called Naruto, the character with the same name will again surpass Sasuke, just as Goku did with Vegeta. Also, because the Uchihas' is so strong, and the story seems to revolve around them(Uchiha Madara was brought up in Shippuuden episode 1 and I'm confident we will know/see much more of them soon), which strengthen my thoughts.

I know many of you don't like this comparison, but you can't deny that there is similarity between Naruto, Goku, and Sasuke, Vegeta.
With this I think that I have enough basis to make the following conclusion:

Sasuke will kill Itachi, Naruto will become Hokage and surpass Sasuke. What is the point with all this? I am really looking forward to see what happens. The friendship/rivaly between Naruto and Sasuke, and the fight of the Uchihas, Sasuke and Itachi(yeah, I'm an Uchiha fan:amuse).

You all look forward to this, right?

--> Itachi, Naruto and Sasuke are the main characters.