Tja, Symbian in itself wasn't a problem (the architecture and feature completeness is quite nice!), but the GUI was horrible after 2006 (even i didn't find in the menus what i was looking for) and the CPU power / RAM capacity given was a joke. Anyone remember "Not enough RAM" in Garmin Mobile XT, even with nothing running amongst?

But this doesn't take into account: internal Nokia politics, the unmaintability of Symbian code (remember that article about reverse-enginering their own stuff?), the fight between different business units / factions (Maemo vs. Symbian folks), too many management levels, ...)