Here's a lovely analysis of the Battle of Hoth by Spencer Ackerman over at Wired.

Vader jumps into the Hoth system with a handful of Star Destroyers; only six are shown on screen. That’s got to enforce a blockade of an entire planet. His major ally is the Rebel energy shield itself, which bottles up a Rebel escape to the Ion Cannon’s line of sight. But Vader doesn’t seem to realize the shield’s ironic value. Once Vader orders the shields destroyed, he lacks the force to prevent a pell-mell Rebel retreat.

What he said. Both the Empire and the Alliance look like paragons of military incompetence if you actually think about it. Fortunately, I don't usually think about it. Because Star Wars. *g*