What are the consequences of flying GA without a license (certificate)?
First, if you fly privately without a license can you avoid trouble with the FAA? Yes, if you don't attract attention to yourself.

It's essentially the same as driving a car without a license or insurance: it happens because some people just don't see why they should have to get a license, and it's possible because no one regularly checks every car and every driver. Anecdotally, there are plenty of pilot stories about unlicensed pilots in rural, remote areas who fly out of private strips or small, uncontrolled airports. Alaska in particular is notorious: I've heard the 'joke' several times that the FAA used to have the goal of getting at least 50% of Alaskan pilots certificated.

Second, if you do get caught by the FAA or by law enforcement, what happens? As you said, the FAA can revoke your certificate but state or federal officials can go after you with criminal charges. This is from the criminal indictment of someone who was accused of lying about his pilot's and mechanic's certificates:


  At sentencing he faces a maximum of five years in prison and a
  \$250,000 fine for each of the two criminal counts of making false
  statements to the FAA, and a maximum of three years in prison and a
  \$250,000 fine for each of the three counts of flying without proper
  authorization.


I couldn't find out what happened with that case, though, so I don't know if he was found guilty or not and penalties - if any - he really faced. @mins found another example that did result in a prison sentence.

Finally, there is one legal way to fly in the US without a license: fly an ultralight under part 103.