I have the print edition of this month's Kiplinger's, and there's an interesting sidebar box in the magazine that I don't see in the version on the web about why they only ranked the ten brokers they did:

"Who's In and Who's Out

We started with a list of 25 brokers and removed firms that focused mostly on options or on superactive traders. That left us with 15 candidates.

Several companies that we included in our February 2011 ranking story ("The Best of the Online Brokers") are not present this time around. Vanguard said it was a mutual fund company with a brokerage business offered as a courtesy. Some firms--Muriel Siebert, USAA, and WellsTrade--begged off inclusion in our survey. Interactive Brokers said it "largely caters to professional traders." Still other firms are missing because they've been acquired. Charles Schwab bought OptionsXpress in September 2011. And TradeKing and Zecco were finalizing a merger as we conducted our survey over the summer."