I also like how every season focuses on a particular aspect of the city. S2 with slow death of the middle class/industry (and unions), S3's take on the drug war, S4 on schools, etc. When I heard S5 would focus on the media I assumed it would be a really strong season, given it was straight up Simon's alley. It's not a "bad" season of television by means, and I think some of the criticism is overblown. That being said it features quite a big drop in quality and introduces some rather bland characters. But the last episode is great IMO.
Also another thing I love about The Wire: nearly every season features a stand out performance from a new character, or a character who had a minor role previously. Frank Sobodka in S2, Bunny Colvin in S3, and Gus Haynes in S5 (the Baltimore Sun editor). All those characters really became the center of their particular season's theme.