Even though this is Game of Thrones, he got stabbed pretty badly. This is exactly why everybody was taken by surprise: they staged it like a horror film, with the villain raising from the dead.
He was stabbed twice in the chest, with the second time an Oberyn literally jumping over him with the spear. We saw him laying down on his back, the spear deeply thrust into his chest, for like a whole minute before Oberyn took it out. The spear blade is like a forearm long. Someone studying anatomy would tell you that his pectoral muscles would have been significantly cut, preventing him from raising his arms. Significant arteries would have been cut as well. Most likely the lungs or the trachaea would have been badly damaged, so if still alive he would be trying to catch his last breath. But it doesn't even matter, anybody without that knowledge but with basic common sense knows you don't get back up with such wound. Hell, a single tiny bullet in the chest takes someone away.
They pulled a cheap trick on us just to make a shocking twist.
I was OK with Ned because he was killed by the words of a twisted mad king. Infuriating but understandable, hate was redirected towards Jeoffrey.
I was OK with the red wedding as it was basically a trap, a betrayal. I didn't see it coming but it was still logic in regards to how the world works. Bolton is an ass but he is so consistently.
I'm not OK with Oberyn death because it was by someone raising from the dead to give the ultimate blow, only to die again. Had it been orchestrated any differently, I would have been OK with it.