If you have the money and want to upgrade, do it. Help the economy, you will be happy, its a win win. Personally i dont think you need to upgrade anything, i test rode the defy and really loved it. I have tiagra and it works flawlessly and i abuse it and it still works well.
But if you "need" to upgrade, upgrade comfort things. Start with the contact points. You have a brooks already so try wheels and tires. I went from the stock vittoria zaffiro's to mavic aksions and they are so much more smoother and take bumps better. Some new bar tape. a new carbon crank.
Get a garmin 500 with cadence and HR, they are fun.