The game is meant to be accessible to everyone, after all. You friends can easily jump in as instant contenders regardless of skill level. Certain games that declare winners without any rhyme or reason will give some players reason to gripe. As will the game's tendency to charitably level the playing field via cheap spinning wheels and bonus stars. But what really sucks most about the transition is how much better we've already seen it done before, on its home system no less.
Perhaps we're a little ADD, but Mario Party 8's biggest flaw is that it moves entirely to slow. You'll have to read layer after layer of instructive text no matter how many time you've played. It all gets very repetitive. We weren't all raised on Teletubbies, and even the ones who were aren't nostalgic for information and scenes that happened 10 minutes ago.
Every piece of attribute-boosting candy consumed is accompanied by an unimpressive cutscene that runs a little too long, and you'll have to view them every... single... time. Sure, you can turn off compute opponents' cutscenes in single player, but you'll have to endure a deluge of crap before you can get to the action. A 15 turn, four player game takes over an hour to play through. Given that most minigames are around 30 seconds long, this means you'll get a little less than 10 minutes of partying per hour. A shame, because board play is where the multiplayer has always shined brightest.