Been playing most the afternoon. Here's a tip for you though, don't play multiplayer and expect sanity if your friends are jerks. One of the most insanely fun times in front of my Wii I have ever had. My cheeks hurt from laughing.
I agree, I definitely get a little crazy when my roommate does something insane. He seems to be a bit out of it, as of late, but for a little while at the end of World 1 through the middle of World 2, he had a thing for picking me up and just throwing me wherever he felt, which could include into a deadly enemy, among things.
For those of you who have watched videos online released by Nintendo, those sort of things are the primary unlockables, up through all the worlds. I don't know what adds the videos to the list that you can buy with Star Coins, but I know on my "Group" save file, we've got 13 extra coins with nothing to spend them on, and we're not playing perfectly, coin-count wise.
Some videos are nearly unreplicable, and are just stunning. You've probably heard mention of this. Some are just stupid, for instance there's one involving a star on world 1-2 titles "Infinite Lives," and in it, the player gets a star, and kills enough enemies to earn five lives, and then the star runs out, which clearly isn't infinite. The last group give genuine "How-to" pointers, such as finding a secret exit to some stage, or locating and nabbing all the star coins in a stage. It's a very good idea, one I truthfully have enjoyed so far

Also, something no one has mentioned, as far as I've seen: You know in multiplayer that when you die, you float back in a a bubble, and you also know that when you all are in bubbles, you lose and have to exit the stage. Did you know that simply pressing the "a" button instantly bubblefies you, allowing you to reach things normally you'd die to get, or just generally annoy the other people? You don't lose any lives or any power-ups, so anyone can do it, just make sure you don't do it and let the last people remaining die, or even press the button themselves! While you don't lose a life if you end up leaving the stage, you will lose any progress aside from what was achieved up to the checkpoint, as well as whatever power-ups you had at the time this happened.
And, just to mention, when you get a Toad up to 99 lives, nothing special happens. Whether or not something happens to Luigi, I haven't found out, either.