My guess is that much of its popularity came from the fact that it was the first of its kind, a show that followed a large cast of characters and points of view at a fairly slow pace, giving equal time to the investigative arm and the drug sellers side (both portrayed with equal sympathy). It set up a path for later shows that borrowed from the idea that single episodes weren't stand alone pieces, but tied together into more and more intricate story lines of a big cast, with no single character fixed at the center. Having seen some of those inspired programs first and this series later, it's hard to be blown away by the narrative. The pace is slow, and we're often confused by what's actually happening, especially with the various lieutenants and majors and officers and politicians doing underhanded deals, and the turf wars among drug sellers. The tech end of it isn't compelling, because it's basically about listening to phone calls, which may have been novel in the early 2000s, but is less fresh now. And for me, there's also the fact that you have only two major female characters (a cop and a district attorney), and the rest of the cast and story lines revolve around dudes. For some reason, though, we're reluctant to give it up; still moving forward under the persuasion of other viewers who loved it, but watching it seems more like a dogged commitment than a pleasure.
I'd love it if someone who watched the show when it was on would go back and start it again to see if it has the same power today as it did when it first aired. Adam??