Something that I've noticed that somehow ends up lost when people learn "the model" is the encapsulation aspects.

I don't know if it's missing in people's course work or what, but I've had to use http://www.tcpipguide.com/free/diagrams/ipencap.png many a times to explain how stuff like VPNs work, correct statements like "firewalls don't have a routing table, firewalling is layer 4", explain things like MTU and payload size, or why certain traffic doesn't go beyond a broadcast segment normally.

Personally I think this is one of the better visualizations.