Steven Hayden's Leno postmortem hits the nail on the head:
Just as The Simpsons did around the same time, Letterman confirmed that you were smart and “correct” for believing that mainstream pop culture was imbecilic. A person like Letterman trained you to see through people like Leno. He made you want to be the guy who silently mocked the other guy for trying so hard to fit in.
Leno is the nerd who learned magic to pick up girl at a party (but never went) and Lettermen nerd who went to parties drank someone else beer, smoked someone else's pot, then called the cops on the way out the door. He also love drums.