The Wire was special in several ways: First was the dialogue, especially the Baltimore accent and slang. I never even heard anything close to that. I'm in Europe, BTW, and even with subtitles I had a hard time following (that was before urbandictionary, mind you). It was actually years later that I learned Idris Elba is British, which still blows my mind.
The characters and casting are impeccable. Idris Elba, Dominic West, Lance Reddick, Wendell Pierce, Wood Harris, all absolutely incredible and pitch perfect in their roles. Probably most memorable is Michael Williams as Omar Little, a side character in the show which is impossible to forget.
It's not a perfect series, but I'd say the first season is damn near it. I would even go as far to say that it's probably enough to watch the first season to get most of what this series wants to say, mainly to watch the tragedy of the war on drugs from the deeply humanist perspective of David Simon. IMHO the later seasons never matched the first one. On the contrary: the later seasons usually have one central theme which is explored (like journalism, education, etc.) and get a bit preachy at times. Also, the series lacks character development. Yes, people become more cynical and burnt out, but they are already very cynical and burnt out to begin with, so there's really not that much to build upon...