They made a -lot- of stupid decisions, both in sticking to 'what they knew' and bad decisions with cutting-edge tech (N900 comes to mind, you couldn't get one in the states with the right bands for 3G).
I will always love the Lumia cameras however, even their shit tier models had great image quality.
They should have continued maemo 5 and bet hard on it. The big rewrites for n9, and continued focus on symbian as a cash cow, hurt them.