I came across an excellent article today that goes into a lot of detail on the Symbian OS and the various phones that use it, and the various factors that have conspired to cripple it. Give it a read here, and find out why the iPhone is likely to leapfrog this formerly great OS in short order.
My own take on this is that the article hits the mark pretty well. I had been investigating new phones for some time to replace my old Nokia 3650, one of Nokia's original Series 60 units, and was fairly disappointed with the newer Series 60 OS. My old apps wouldn't work, and suitable replacements were not to be found. Looking in from the outside, the 'new' OS made little sense from a consumer standpoint, the Symbian OS was in my opinion by far the best OS on any smartphone, but management decisions were killing 3rd party development, and a bloated OS was being built on what now appears to be a fairly shakey foundation.
The Symbian phones were succeeding despite themselves, simply because of the fact that everything else sucked more. The new iPhone with a core OS of OS X is indeed years ahead of other phones. With its core OS being based on OS X Leopard, the latest and greatest features and code are going to be available to developers who already have experience building Mac applications. The iPhone will debut years ahead of where other phones are heading, once developers get on board and Apple itself expands the feature set, this thing is going to be unstoppable.