A (very) small number of The Wire's major characters.
The reasons for The Wire's success are fairly straightforward: excellent writing based on real police procedures and investigative techniques; strong and memorable characters; and a thorough exploration of the systems and institutions of the modern American city, both legal and criminal. It also had a biting, black sense of humour. The show also never pulled its punches. If a storyline required the death of a long-running major character, they would die. There was no filler and even minor, almost glossed-over details could turn into major revelatory plot moments several episodes later. The show also changed its focus every season to retain a degree of invention and freshness each year, whilst retaining a core cast of characters. Ultimately the show's message was that life goes on and the systems meant to help people rarely do so, sometimes making things worse, and that even the most well-meaning reformer will inevitably get trapped by the system and their ambitions quashed. Despite this cynical message, the show also showed that people will go on and live their lives the best they can, despite and often in spite of their circumstances.
As part of the 10th anniversary celebrations, Maxim has an excellent article here featuring contributions from several of the writers and actors who worked on the show. Well worth a look.