Almost no flights are held back for connections.

There's probably the odd flight that is held back if there's a large number of passengers that will miss it due to a late connection--and there's no later flights with seats available. I would imagine the cost of holding the 11pm LAX-SYD flight because 20 people are 10 minutes late on the NYC-LAX connection is a lot less that paying for hotels, especially when the next flight may not have 20 seats available anyway. Though then again, most carriers have gate pressure at LAX and they might not be able to hold the flight anyway.

It's an interesting optimization problem to solve!

From what I understand, all flights across the nation were equally impacted. My connecting flight was delayed, but ended up leaving right before my flight landed.

Interestingly, there was a very, very long line at the ticketing counter of Delta this morning. I asked a few passersby if they were put up at a hotel like me. Every person I asked was in the same situation.

It depends on what the destination is. I sat on a plane for over an hour at the gate waiting for a delayed flight with passengers connecting to my flight. This flight was the final leg for everyone, and the pilot was going to be able to "make it up in the air". So, I'm guessing they have ways to decide if it is worth delaying a flight to wait for another delayed connecting flight.