A JMN is (almost always)a grown adult who dares take plastic models seriously; having forgotten what was fun about it to begin with. They have all the resources and knowledge in the world to become respectable hobbyists, but have the creativity of a free-range garden slug.
JMN behavior can be broadly described as anal retentive, and can stem from insecurity resulting from having a small penis.
Anyone who can sit through a seminar about Messerschmitt] Bf 109s without falling asleep
A guy who buys more than one Airfix TSR.2 but will only build them in historically accurate paint schemes.
The snooty clerk at the hobby store who faints when you buy a rare OOP Aurora model kit from the 1960s only to chop it up and turn it into a flying submarine robot.
Any jerk at a modeling club who says contests are for "Serious Modelers only"(meow)
The nerd who collects video stills from every possible angle of the USS Enterprise-D in order find out where the septic tanks flush out and modify his AMT kit accordingly.
Messerschmitt Me 109 vs Bf 109.
Expensive aftermarket add-ons for kits.
F-16 Viper Block number.
Deficiencies in commercially available models.
Details that would be impossible to see in miniature, such as a pilot's eyebrows.