Investment Pains Good For Diversification?

An article in the Wall Street Journal (link on Yahoo! Finance) titled Failure of a Fail-Safe Strategy Sends Investors Scrambling by Tom Lauricella (7/10/2009) makes in interesting point about why investment asset classes that were traditionally considered non-correlated now move in tandem:

"Prior to this decade, investing in commodities was a complicated process due to the complexity of the futures markets."

ETF's and ETN's now make difficult-to-invest-in assets easy. So perhaps the takeaway is that asset classes like commodities were great portfolio diversifiers because they were hard to invest in. So... what's left that's still a pain to invest in?