I'm assuming by "Joe 401k" you mean someone putting a chunk of their income into an index fund occasionally and keeping it there for the long haul. All of the i

I'm assuming by "Joe 401k" you mean someone putting a chunk of their income into an index fund occasionally and keeping it there for the long haul. All of the innovations in the market are good for them without a doubt. Computers can price index ETFs perfectly vs. their constituents so you always get a fair price. Spreads and commissions have never been lower. Wholesalers make things even better for the retail buy and hold guy by filling them inside the bid/ask spread. The kickbacks they pay to brokers mostly translate into very low brokerage fees. Now you can trade stocks for a few dollars at a penny spread but it was $50 or $100 in commission years ago, with spreads of eighths or quarters, which can really eat into your savings when you're investing a small amount. The main things I'd say to be wary of are: a.) Trying to compete with professional intraday traders. Even if you have a good feel for the market, someone faster and smarter is going to beat you to the punch most of the time. The markets are much, much tougher for day traders, and the complex structure makes it easier for sophisticated automated traders to take advantage of them. It's just a waste of time and money. b.) Trading when the markets are in a crisis mode. Automated market makers don't have as much capital as the big banks did because there's simply less "juice" in the business, so they won't be there buying forever in a falling market. No market maker ever would, but when they were earning quarters instead of pennies, there was more incentive to take on risk because the market had a built-in edge. Computers also have a lot of safety logic that can make them leave the market due to bad data during extreme events. You should always set a reasonable limit price on your orders, avoid trading around volatile times like the open or economic releases, and avoid market stops or other order types that can make you trade in an illiquid market at the worst time. c.) Putting your money into things that are either risky like individual "hot" companies where you're competing with the best hedge fund analysts or products that are inherently bad to buy and hold (leveraged ETFs, commodity/volatility ETFs often have time decay or lose money rolling futures).