Rather than watch the last night TV I usually keep up with the parts I like by reading the Late Night Political Jokes. All the snark, no interviews with idiots, no annoying bands you wish you'd never heard of, etc. However, this gives me a unique perspective on the jokes because although I can imagine their delivery, I'm really enjoying the writing.
Comparing current Leno to Tonight Show Leno I must conclude: Jay left the Tonight Show because the writing was too good.
1/13/10 Update: Reading some articles about the NBC Leno/O'Brien controversy it looks like O'Brien signed a contract five years ago to begin hosting the Tonight Show in 2009. Also, admittedly it looks like the writing on the Leno Show got much better after it settled in. In all their wisdom the NBC executives promptly canceled it... Then in an even brighter moved decided to screw over my Irish-American peep. We don't take that well. #TeamConan