I like to think of my collection as diverse (2 1970's Gottlieb EM's, 2 1990's Williams DMD's three very different themes by 3 different designers) but there are strong similarities - all are original themes with objective-based gameplay and shooter rods (as opposed to launch buttons) and none have a "standard fan" layout.
It took me a while, but I figured out what I like in a pinball machine.