WMG / Law & Order

The New York Police Department is secretly run by Shane McMahon
This theory comes from a couple of posts on the tv.com message board for the episode "Avatar." The poster's theory about Shane O Mac belongs in Wild Mass Guessing.

Jack McCoy is a Paladin.
His special mount is his motorcycle. His "occasional moral lapses" indicate that even though he's a Paladin, the cynicism of the Law & Order Universe, he's fallen every now and then... but that's what the spell Atonement is for.

Detective Fontana was dirty.
This was implied by the writers — he seemed to have way too much money — but never explored.

He could've been a Non-Idle Rich; but given L & O's attitude toward the rich, he would've been dirty anyway.

Perhaps he was dirty in Chicago, and came to New York to dodge Internal Affairs.

Jack McCoy is the ancestor of Dr. Leonard McCoy.

Nora Lewin was Dick Wolf's Take That! to critics
His way of saying to those complaining about the show's liberal bias, "You think McCoy's a liberal? You ain't seen nothin' yet!"

Detective Briscoe used to be corrupt.
Back in the seventies, he was on the SIU squad under Detective Ciello. He was prosecuted for corruption, but was acquitted, and remained a detective in the NYPD. All the negative attention, however, drove him to drink. He eventually cleaned up his act, and legally changed his name from Gus Levy to Lenny Briscoe to get away from the ongoing stigma.

The cast of this show got a surprise when Revolution happened.
They found themselves having to get out of the city, and then having to survive the ensuing chaos. Then some of them get the idea that since "law and order" are effectively dead, they can go after all the guilty people who walked away free and make them pay.