1] OS felt dated. Failure to come closer to what customers wanted on a timely basis.
2] Poor app developer experience.
3] A lot of different hardware specs against which to develop apps.
4] Not enough apps. Missing key apps.
5] Slow to innovate after settling on major patent case. The company became very lawyer focused which slowed innovation to a crawl.
I'm sure I'm missing tons of stuff and this would be a great case study, but as a former employee this is coming from my perspective.