Symbian was also designed with integration with hardware in mind and providing core API's for manufacturers and carriers to build a UI on top of. This is why the early owners of Symbian were a plethora of manufacturers and operators. Symbian even tried to designate 3 specific UI frameworks (touch, candybar and QWERTY) named Quartz, Pearl and Crystal.. but the shareholders revolted and went their own UI way (hence S60, UIQ & MOAP).

Disclosure: I worked at Symbian between 1999-2003