Tom,
I think you're fairly accurate in your observations. There seem to be 3 broad categories of modelers; Generalists - who build just about anything, Genre Specialists - who primarily build one type of model (armor, A/C, etc), and Super Specialists - who focus primarily on a specific genre, nationality, and time period (for example Allied Vietnam Era Aircraft). There are varying degrees of specialization within these three categories and you could probably find more than 100 individual "nitches" within our hobby. Also, it's not uncommon for modelers to switch between areas of interest throughout their modeling lives.
For me, the primary nitch is WWII Military Subjects (armor, softskins, guns) from all nationalities in 1/35 scale. While I will build something different every once and a while, 90% of my builds fall into this area.