Is there any particular reason why I never see the MOSFET between the cathode of the LED and Vin? Assuming a Vin of 12V, and a 5V logic signal, should I expect the MOSFET to not conduct from drain to source? Is this why the switch is typically located between the anode and ground, after the load does its business?
Seems trivial, though I never thought the sequence of components in series mattered.
To conduct the gate needs to be +5V w.r.t. source. When the MOSFET is on the drain
and source are at the same voltage (nearly). If the drain was connected to +12V you'd
need +17V on the gate, rather than +5V. This is called source-follower configuration
and is not normally used for switching (*) due to lack of a 17V supply!
(*) Actually it is commonly used for switching in most industrial applications, but this
requires use of bootstrapped floating gate-drivers that work at the same potential as
the source, which itself is pinging up and down as the device switches. Doing this
allows an H-bridge or 3-phase inverter circuit to use all n-channel devices which are
superior to p-channel devices (about 3 times better carrier mobility).