Bitey, the fake serial killer? Super-Omar jumping out of the window? "The Dickensian aspect"?
The middle seasons of The Wire were about the city of Baltimore. The first season was an excellent long-form procedural.
By the last season, Simon seems intent on retrofitting a structure of "each season is about one failed institution" --- the police, the working class, then politics, then schools. For the last season, Simon chose the news media, a subject dear to his heart. But whether newspapers just make for boring TV (see _The Newsroom_ for more evidence) or the story arcs from the last seasons just brought too much baggage, the season was unbalanced and disconnected. Each season involved some degree of suspension of disbelief, but by the last season The Wire was trying to do so much that it was impossible to ignore how silly the whole serial killer plot really was.
Actually, Omar jumping out of the window to escape an ambush was something that one of the real-life guys he was based on (Donnie Andrews) actually did. The only thing that was made up about that was in the show they had him jump out of the fourth floor because they felt that people wouldn't believe it if they showed him doing what Donnie actually did - jumping out of a sixth floor window and limping away.