Re: I've been waiting a long time for this.
My biggest problem with it is the UI (which I suppose you could argue is an "App" issue rather than an OS issue as such... ) The bones of symbian, the call quality, the battery life, the memory management - those are all very good but it has always felt like the S60 UI was a bit of an afterthought. Something knocked together from leftovers of S40 rather than something properly designed for touch control. It's come a way, since then, but there's still a sense that they didn't really try as hard as they should have and it's a lot less good than the competition. I don't know if that 640x360 resolution was a technical limitation or just a way they could cut the development budget by reusing stuff from the C6 but for something at that price point it really wasn't good enough.
Perhaps symbian could still be a serious player if it'd not been killed off the way it was - after all, the first couple of versions of android were bloody terrible, but that's gotten really nice now. What I want from a "modern" phone OS is something that handles all the "phone" stuff as quickly and easily as possible. iOS isn't bad, I think the current iterations of android are better, and as it happens I like WinPho better still... but I don't think any of the three are _bad_ as such these days.
It's a tough question to answer outright, except by example. I'd argue that winpho is a "modern" os, that android is, that iOS is aging a bit, but is still OK. Hell, I liked WebOS, thought there was a lot of potential in that, but the poor thing never really had much of a chance.
In an ideal world I'd take Winpho's ease of use, with Android's customizability, iOS's stability and Symbian's efficiency... doubt I'll get it tho.