The financial meltdown has led to only a few civil and criminal cases against executives, and even those focused on peripheral issues: Goldman Sachs’s peddling of a credit derivative obligation and the communications of two former Bear Stearns hedge fund managers. But the Securities and Exchange Commission’s securities fraud action against Angelo R. Mozilo, former chief executive of Countrywide Financial, promises to feature the aggressive mortgage practices of what was then the nation’s largest mortgage lender.