USA Today has an article listing the top reasons why GM failed, but they miss out the key factor. For too long, GM was profitable only because: (a) they financed their own deals with GMAC (b) they made up in truck and SUV profits what they lost on cars. This may not have been a problem, except that it was exacerbated by the terribly short-sighted incentives given to American managers.
When GM's profits started coming from financing and monster trucks, the company started to throw money at their profitable units and start neglecting the "cost centers". The "cost center" in this case happened to be the raison d'etre of the company and so, GM lost its core competence.
The other reasons listed are all valid, but the key reason that GM is bankrupt is that GM, long ago, ceased to be an engineering company.