I think to answer the initital question....just look at how pilots go about their training....single-prop PA28-type with 160hp then a twin prop, a sim ride and then a 737. It's a bit like getting a nice big 7-series after learning to drive in a Suzuki Swift or something I guess.

And...

the ignition key is gone.

regards

Besides the points already mentioned, one of the biggest hurdles I remember were the much shorter stage times on flights ie. the time between one waypoint and the next, which meant having to speed the thought processes up to stay ahead of the aeroplane.
I think my brain finally caught up with my a_hole after about 6 weeks, during the initial jet conversion (from F27 to DC9).

The extra honeys down the back were another distraction that compounded the conversion.
(That might be the Phase 4 Old Smokey is looking for - can you imagine the variety on an A380 := )

to quote a SR-71 driver -
 

"You ain't been lost, until you been lost at Mach 3"

Barit 1: the guy ("fast Eddie") who set the "official ;)" speed record in the SR-71, described on a plaque at Dulles (IAD), flew several times with me as FO. He went from the SR to flying VIPS around in a Gulfstream III from Andrews AFB.

About his record flight, he is a nice, humble guy.