Season 2 is probably the most important season out of all. Like somebody said above, season 2 main purpose was to build up season 3,4, and 5. Watch season 3,4,and 5 and see how many times they bring up somebody or something that happened in season 2.
I think people just got upset with that season because they strayed away from the streets. Majority of the focus was on the docks. The show isn't about the streets, it's about Baltimore. That second season put light on the blue collar urban workers and how they are going through a plight. That's just as much as a problem in Baltimore as is the school systems, the streets, the cops, the media, and the political system that everybody seems to get caught up in with this show.
What makes the show good is the storyline. Not the k!llings. Season 2 probably had the best storyline out of all of them to me. It just didn't have that build up or hype like the rest of the seasons. But like I said before, season 2 made the show what it is.