During the first season, this was far and away my most disliked show on Thursdays. I stuck with it only because it was wedged between other shows I liked. By the end of last season, it was my second-favorite show on Thursday by a pretty large margin (right behind Community).
The biggest differences are:
- Amy Poehler toned her performance WAY down. During the first season, she was so shrill and criminally clueless that she brought the show to a screeching halt every time she was on screen, which was for most of the episode.
- The writers stopped mocking the characters. None of the characters were really likable during the first season, and watching nasty people get ridiculed isn't a very fun way to spend a half-hour.
When the show came back for the second season, they found ways to make the characters relatable, and in many cases kind of noble, without doing a 180-degree turn on the characterizations. Now, the humor comes from the characters in such a way that we're laughing affectionately with them, instead of at them.
That's a fine line, but an important one -- Andy being revealed as an enthusiastic moron instead of a cynical moocher; Ron's anger and fiery libertarianism being softened by his honesty, loyalty, and fatherly impulses towards Leslie and his staff; April's anti-social monotone being melted a bit by Andy's sunny optimism; Tom's manipulative douchebaggery being offset by his affection for Leslie and unrequited feelings for his green-card wife.
The show didn't eliminate the qualities of the characters that made them unlikable in the first season, but it helped justify them for some characters, and overcome them for others.