Many MLMs have "optional" national meetings, motivational books and tapes, and other various expenses. The cost of these comes out of pocket of the individual. As a general rule, if you can be one of the first people to join a successful MLM, you'll make a lot of money, especially if you can sell all of this crap to your downline. However, if you're not in this lucky 1-2%, you'll honestly be lucky to break even in your expenses.
There is some technical difference between a pyramid scheme, which is illegal, and an MLM, which is legal. However, nobody but the high-priced lawyers which keep MLMs in business and stealing the money of its starry-eyed employees seem to know what this difference is.