Don't forget good humor and brilliant character chemistry too. I think the actor choices are spot on. Baldwin and Buy More morons are funny as hell. :) I even enjoyed Chuck's and Sarah's fake relationship antics. Seriously, this is one of the best tv-series I've seen.
Season 2 finale/Season 3
In the season 2 finale Chuck gets upgraded Intersect into his brain. Which apparently allows him to access pretty much any information, including kung-fu. I'm not sure whether this is good or bad. I enjoyed Chuck's character because he wasn't a super spy, he only had his smarts and the old spy-secrets-only-Intersect. But now I assume he can excel at close combat and firearms too whenever he needs to, which I'm afraid could destroy his character and make Sarah and Casey just sidekicks.
Some of the interviews with producers/writers/whatever (folks part of the show) know that they would destroy the show if Chuck was suddenly the super spy guy. Apparently accessing this information stored in the Intersect upgrade won't come easily.
So I don't think there will be too much to worry about. I seriously cannot wait for the new season to start!