I know that it may not have been Death who spoke to Sam and it may just have been Sam's head being weird, but I had this idea so I decided to disregard the ambiguity and just roll with it. I hope you all enjoy and that I don't break your already post-finale broken hearts even more with this idea.
"Well played, my boy"
Death is inevitable. Everyone will meet death eventually. Some fear nothing but death, some beg for him. Some bring death upon themselves, some bring him upon others. Many try to hide, but we all meet death eventually.
Judgement and morality and good and bad are messy and complicated. But Death is fact, cold and clear. Despite all this, despite dealing with every soul there ever has been and ever will be, despite only being fact and nothing more Death took the time to judge one man. Just one.
Sam Winchester had lived a life unlike any other. He had fought and suffered and struggled all the while hoping for some normalcy, some happiness, a life without monsters and demons and angels.
Death knew this, he knew everything about Sam Winchester's life and believed that this one man deserved a little something more, nothing much, just a few simple words. Words which would have been rather underwhelming from the mouth of anyone but who spoke them. Death passed his own judgement on Sam Winchester and told him that he did well.
Of course, with the Winchesters, nothing is ever so simple.
Death's judgement of Sam could truly be considered the greatest honour of all, and he hoped Sam understood that. Because Death's words were also an apology. Death needed Sam to understand that he truly had been good in life, had done good, and deserved good. Sam Winchester needed to understand this. Because even after the life he had lived Sam was not going to heaven, although no one had ever deserved heaven more than him.
It would seem that even in death Sam Winchester would not yet be in reach of the happiness he wants and deserves.
The gates to Heaven were shut, after all.