This was a feature that I was highly anticipating, but now I think it's gonna suck.  Check out this hands-on article.  It's a little old, so I guess it's possible that some things have been fixed, but there's 2 things that really stick in my craw.

http://www.joystiq.com/2008/04/15/joystiq-hands-on-star-wars-the-force-unleashed-duel-mode-wii/

"These fights require two human players -- there's no AI option..."

WTF!  How do you create a fighting game/mode without a single player option?  LAZY!

"...their moves are all about the same as in the single-player game, so Luke uses the lightning strike alongside the "dark" characters."

Again, laziness!  I don't want Jedi to use Sith powers.  This is gonna blow.

I can't tell if you're being serious.

The duel mode is online is it not.... so you have to be fighting a real person.

And how is adding an ability (lightning power) to the battle being lazy?

IDK it it will be online...

But I think he means they were too lazy to stay accurate to the story and give each character (or take away, in some cases) attributes...

judging a book by its cover?

IDT it's online.

But I find it hard to say it will suck when you haven't played it.

I don't see the point in it, but it may not suck.

Your first mistake is using an old article.

Current reviews of the game are scoring it pretty well, so the problems probably aren't as bad as you fear.  Besides, I never expected the duel mode to be 1-player.  I guess it'd be nice...

 The reason he's saying it sucks is because they won't let you fight against an AI opponent  only other people at your hougsh which sucks and completly ruined the Duel mode which was the only good thing about the Wii version.

its going to rock. be quite.

I don't think it's online. Even if it is, they need a single player duel mode option as well. As for the force powers, there are certain powers that are dark side powers such as lightning that a Jedi would never use.