adiabatic expansion vs Joule-Thomson Effect
In the Joule-Thompson experiment, there is major viscous dissipation of mechanical energy (in this case pressure) to internal energy occurring within the pores of the plug.  This is analogous to Poiseuille flow of a viscous fluid in a tube.  If the tube is adiabatic, the fluid flowing through a tube experiences a temperature rise as a result of the viscous "heating."  The work to push the fluid through the plug is totally dissipated and converted to increased internal energy of the gas.  The reason that viscous effects are so important in flow through a plug is that the pores are very small.  So flowing the gas through the plug is analogous to flowing the gas through an array of very tiny tubes in parallel.  

Ordinarily when a gas expands adiabatically, it cools.  But, for flow of an ideal gas through a porous plug, the adiabatic cooling is exactly offset by the viscous heating.  So there is no temperature change.  And, as a result, there is no enthalpy change.