Badass gay character, humanized gangsters, contra-Reaganesque attitude to drugs, aim at corruption and money, etc. were pretty great for its time, but now that a lot of media has caught up, the lackluster areas stick out a lot more.
It's got liberal sensibilities, not leftist ones.
For example, S2 is about the shipping container women. McNulty swears she won't treat them like a mere blip or statistic, but will remember their humanity. At the end of S2, they have a gag scene where McNulty goes "undercover" at a whorehouse, and gets serviced by some prostitutes, while everyone at base overhearing the mike goes "ooooh youuu McNulty".
I think this shallow and circumscribed commitment to nominal values, not fully comprehending the extent to which they require a fundamental shift in approach, is pretty liberal.
There's also plenty of blaming of teacher's unions for shutting down "innovative" approaches. All the issues chalked up to "capitalism" have to do with individual corruption and not the regular brutal application of market logic to human problems (eg: corrupt construction agencies wanting bribes vs. regular white citizens protecting their interests).
but perhaps the most anti-leftist aspect of them all is how the show is utterly grim and hopeless and nothing ever changes. all radical initiatives are shut down and ruin lives, and it works more as a cautionary tale than an inspiration. if all media is propaganda, The Wire works as propaganda for obedient incrementalism, and decidedly against anything revolutionary.