The Symbian UI & OS separation dates back to its Psion days.
Back then, it was already clear from the docs that Psion's own EPOC32 developers saw the OS and the GUI as separate elements.
Symbian's main issues were:
1. Terrible development tools. No seriously, they *sucked*.
2. Developers usually design and write applications to a particular GUI. The more GUIs you have, the more fragmented the development process becomes and the harder it is to provide decent QA and customer support. ("Are you using the UIQ version? Series 60? Series 80? Symbian^3? ...etc.") It's the exact same reason Linux has singularly failed to make major inroads into the desktop.
3. Qt is a step in the right direction, but it feels like too little, too late. It's also OS-neutral (in theory), so it neither helps nor hinders Symbian as such.
4. Symbian was designed in the 1990s for devices with very little power, and even fewer capabilities. It shows. It doesn't even support proper C++ exceptions, and has some major legacy support issues. It was great for its time, but its time is arguably over.
There's not much reason to choose Symbian over one of the umpteen UNIX clones or derivations out there. It offers little they don't provide, while offering said features through nasty, ugly, convoluted C++-on-crack APIs. Nokia have been making noises about shifting their smartphones to MeeGo—another Linux-derived OS—so who'd buy a Symbian smartphone now, knowing that none of the pitiful number of apps will be compatible with future models?
However, I'd be very wary of writing-off Symbian if Nokia can learn from both Google and Apple. Stronger, more consistent design is needed. Nokia have the resources to take their time getting it right. And their Series 40 GUI is still one of the best low-end mobile phone interfaces ever created.
Android is going to suffer badly from fragmentation if Google don't step in and lay down the law. The OS itself may be quite widespread, but—again—which versions are the most popular? Which form-factor? Is the display resistive? Capacitative? How many buttons are there? What's the resolution and aspect ratio? How likely is a manufacturer to provide updates to the OS? (Not very, by all accounts.) This car is veering unerringly towards Wintel PC territory, where the sheer range of hardware it runs on becomes a curse rather than a blessing.
iOS developers only have a tiny number of devices to test against, allowing for much more time to be spent on design than testing. It also radically reduces support costs, which are a major factor when your app is being sold for peanuts or ad revenues. Apple may not be flavour of the month, but there's method behind their control-freak madness.