That leaves the hardware arena, where, by my reading between the lines, Nokia felt it would not be able to compete effectively enough given Samsung's "vertical integration", not to mention, their "resources" (read: marketing budget).
1) Branding
2) Hardware reliability
3) Customer service
4) Ownership events
5) Carrier logistics and partnerships
EDIT: 6) Industry 'firsts', such as SMS and MMS that were pioneered by Nokia. These days it's Samsung, Google and Apple coming up with new features while others struggle to catch up.
The 'OS customization' is just the potatoes of the business. Most people are just as happy - or happier - without any customization or differentiation on the software level. Windows Phone isn't exclusive to Nokia either, so the sheer stupidity of Nokia's strategy should be plainly obvious.
Or maybe... Could it be that Android has commoditized the handset business, just as Windows did the PC hardware business, so maybe all those things don't help as much any more?
In response to your edit: It's not really Samsung bringing new things to the table, it's Google and maybe to some extent Apple. The big innovations aren't in the hardware or even the software, it's in the services, such as Siri and Now. Sure every Android vendor and their grandmother have their own cloud services, but the ones everybody wants are Google's. Unfortunately, since it's not limited to a specific Android manufacturer, it wouldn't help Nokia either.
Is there any evidence of that? From what I could Google, Windows Phone market share is still under 5% and only recently surpassed Blackberry: http://www.idc.com/getdoc.jsp?containerId=prUS24108913
I think that's a good demonstration that number of people determined to buy Windows Phone in the market is, in fact, entirely trivial.
Nokia could have, and probably should have run Windows Phone, Android, and Meego on their smartphone hardware. And the likely result would have been that WP would not sell well compared to the other choices.
EDIT: The parent post here sounded just a little too pat. But I didn't check until after I had posted a response. Yup. It's this first post even for this user. Hello Nokia PR!
They were right to predict Samsung would be the major Android player.
Instead of competing against a company that already had a major lead and essentially becoming one of the smaller fish in a larger Android pond, it's taking a bet on becoming a large fish in a smaller pond that has very real upside potential. I stress potential because we've all been a little disappointed with Microsoft's latest maneuvers, but there's still a chance for the company to turn it around.
At the end of the day I respect such a great risk being taken at a massive firm (you don't see a lot of that these days). Nokia still believe a third ecosystem can be a very real option for smartphone consumers and they want to be that third option. Takes guts. I'm all for a little more competition.