Draw 2 cards.
I don't see why this is so game-breaking that it has to be banned. You just get cards. What's the big deal? Heck, the anime uses it all the time (especially Yu-Gi-Oh! GX, where Jaden and/or his opponents use it in nearly every episode), and it doesn't seem to cause problems there.
Most of the cards on the Forbidden list are clearly there because playing them makes it very difficult, if not impossible, for your opponent to win. The rest of them seem to be there because there is a loophole in the rules that creates an unintentionally powerful effect. But neither of those seem to apply to Pot of Greed. You won't necessarily win by just drawing two cards, and there aren't any loopholes.
Unfortunately, the Internet has been less than helpful. I just seem to get forums where people say that it's an incredibly broken card, but don't explain why.
The only thing I can think of is that maybe having three of them in an Exodia deck makes it too easy to win before your opponent can do anything. But even then, I don't see why it must be banned instead of limited to 1 or 2 copies.
So could someone please explain to a casual Yu-Gi-Oh! player why this card is banned?