Well, I guess this has become the unofficial Mario Kart thread so I might as well add these new and fun little details on the game. The more I hear about co-op, the more the game just sounds better and better.
Discussing Battle modes:
Bill [MK: DD rep]: "....Balloon Battle has always been alot of fun, that's kind of the classic." "..You can play co-op Balloon Battle, where one's controlling the driver and one's controlling the guy in back. The guy in back can drive by and punch other karts, and when he does that he steals a balloon."
Fran [IGN editor]: "Oh, so if you lose a balloon you can grab one back for your kart?"
Bill [MK: DD rep]: "Yeah, exactly. So you could be down, like - somebody could have 3 balloons to your 1, and you could swing by, punch them, they lose their balloon, and all of a sudden it's tied 2-2." "So they kind of bumped up some of the gameplay in the standard Balloon Battle. And then they went and they added the Shine Thief mode and the Bob-omb Battle. Both of which - I mean Shine Thief is kind of like the inverse of tag where one person is 'it' and everybody else is trying to get them. And you basically want to hold the Shine sprite until the time runs out." "...And then there's the Bob-omb mode which is ::breathes in an looks at Fran::...
Fran [IGN editor]: "It's crazy."
Bill [MK: DD rep]: "It IS crazy. It's totally crazy. And actually, when we got Mario Kart in the office for the first time back a few months ago we were playing it. You know, everybody kind of sat down and got into the 1-player and was playing the game and we were like, 'Wow, this is great. The control's great, it plays great.' And then finally after, you know, an hour or so, we're like, 'Ok, let's check out this multiplayer mode and see what it's all about.' And we cracked it open and we're like, 'Hey, [surprised look] what's this 'Bob-omb Battle' thing?' We jumped on that, and it was like, that was was the rest of the day." "The thing that amazed me was, I mean really, the Balloon Battle in N64 was so much fun. You could play it for so long. And it's like, all of a sudden you have Balloon Battle, and it was like this 'thing' that was the most fun ever it not even nearly as fun as ::laughs while saying this towards Fran:: the Bob-omb Battle."
Fran [IGN editor]: "It's a matter of preference, but yeah. We played Bob-omb Battle and it's really really fun."
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Discussing co-op modes:
Bill [MK: DD rep]: "I think it's been a really long while since we've had some serious co-op action.." "I think people are kind of looking for it." "...I know people are thinking, 'Ok, co-op mode, the guy in back, all he does is sits there and he throws items and if he doesn't have an item, he's got nothing to do,' which actually isn't the case. You got the L and R buttons so that all you have to do is pull up next to somebody and it's like :

unches his hand:: BAM!" "...The guy in front can press a button to toss the item to the guy in back."
Fran [IGN editor]: [surprised sounding] "Oh, really? How does that work, I haven't done that at all yet?"
Bill [MK: DD rep]: [explains to stupid Fran]
Bill [MK: DD rep]: And also, the guy in front is controlling the power slide where he'll hit the L or R buttons to initiate it, and the guy in back is the guy in charge of getting you the turbo boost ::mimicks tilting the control stick back and worth:: out of the power slide." "And say the guy in back has a special item that, you know, you're in the lead and you want to hold on to an item incase you're thrown back to third place or something, [Fran and him start talking about items out of nowhere]." "And actually, you guys haven't played with Princess Peach, have you?"
Fran [IGN editor]: "I have, as a matter of fact."
Bill [MK: DD rep]: ::Looks at camera:: "I kind of expected that from Fran [Fran is a guy]." "Anyways, her special item is hearts. The hearts revolve around the kart so that if any item hits the heart, she basically picks up that item and can use it."
Bill [MK: DD rep]: "So anyways, [continues from where they left off about 'holding on to an item incase you're thrown back to third place'] if you're playing co-op on the same kart and you've got this item you want to save for later, both players can hit the Z-button simultaneously, and that's how you switch spots." "Especially, I mean - I was watching you and Matt [another IGN editor that's filming the interview] play, and I know Matt wasn't very happy with your driving skills and so he kind of forced you guys to switch spots, and he took control of the wheel."
Fran [IGN editor]: "Yeah so - co-op is really cool. You can do it for Grand Prix, battle modes..."
Bill [MK: DD rep]: "Yes, you can do it for battle mode. You can even do it for LAN mode." "...So if you have the LAN...you could have a severe Mario Kart LAN party, 16-people simultaneously."
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Discussing Online:
Bill [MK: DD rep]: "It's not a question of not being able to get there. I mean the LAN technology is the same as the online technology." "Really, Nintendo's issue is, we don't want people who just payed a few dollars for our game to then have to pay us more money just to play the game." "Everybody seems to be talking about online as, you know, the next big wave in gaming. And yet everybody only seems to take a game's multiplayer and make it online. And that's all they're doing and nobody is really looking at what kind of innovation they can do with the fact that online is now doing something different with connecting distant players. And with LAN, it's just proving to people, "Hey, we have the technological capability to do online if we want to. We're just waiting to do it in a way that's going to make sense for us, and make sense for the user."