They should have partnered not only with Intel, but with Palm, RIM or whatever other then-giant to rival Android. Those two went their own ways with WebOS and buying QNX, so maybe they could have agreed to form a consortium for an open and interoperable mobile OS
I loved my N900, and my N800 before that, and I would have loved to have seen successors. Ultimately, I ended up switching to Android because I was tired of things only available as apps. Since then, web technologies have gotten better, and it's become much more feasible to use almost exclusively websites.
Hit ctrl-f and typed Meego as soon as I saw this thread, hoping I'd be the first. Alas.
The N9 was literally a vision from an alternate timeline where a mobile platform from a major manufacturer was somehow hackable, polished, and secure. Favorite phone I've ever owned and I used it until it started to malfunction.
Had a Jolla for a bit, too. It was nice to see them try to keep the basic ideas going but unfortunately it was a pain in the ass to use thanks to their decision to go with a radio that only supported GSM/EDGE in the US. Had to carry around a MiFi just to give it acceptable data service.
I think the idea with Jolla is that if Nokia ever did an about-face, they were ready to be reabsorbed and get things back on the right track. Unfortunately, though we do once again have a "Nokia", it's just another Android white label with no interest in maintaining its own leading-edge smartphone platform.
When I saw the title, my first thought was also MeeGo. While I don't believe it would have been all that great had it not been abandoned, MeeGo absolutely should not have been discarded in such a disgraceful manner.
I was gonna say Meego. They killed it just as it was getting to a usable state. One of the last chances we had to get a proper third option in the mobile market.