With their low fees, all-day trading and tax efficiency, exchange-traded funds have captivated investors. There were no ETFs before 1993, and only 80 in 2000. By the end of 2006 there were 359. Today, there are nearly 700, including 280 launched in 2007 and 30 through mid-March this year. They hold about 0 billion. While most experts think ETFs were a good innovation — built like index-style mutual funds but traded like stocks — some worry that the increasingly specialized ETFs introduced in recent years stray from the faith, encouraging too much risk-taking. That concern is heightened for some by recent Securities and Exchange Commission proposals to let new funds come to market with less oversight, and to invite proposals for introducing actively managed ETFs. Like actively managed mutual funds, these would employ teams of stock pickers and analysts trying to beat the market’s gains rather than simply match them, as today’s ETFs do.…