The rich have enough money, so they can afford to take chance.
If people in the middle class start taking those kinds of chances, most of them won't end up rich, but most likely end up poor, losing their nest egg.
Diversification is still the best idea for most people -- you won't "make a killing", but you'll experience steady growth and protect your nest egg.
You're absolutely right; slow, steady growth, just like saving money from each check in the bank, is always a good thing.
Well, I'd take 10% a year compounded [historical return on S&P500]. Everything in moderation, even greed.