If you haven't watched up to the show: SPOILERS SPOILERS SPOILERS.
Alright, first of all, I HAVE read the books, and when I read that part, I literally put the book down and didn't come back to it for a day or two. I agree that this character was my favorite to read about. He was fun, he was a master of both intrigue and fighting, and he was interesting.
What I figured out after his death though, was that he had one major weakness: he KNEW he was all of those things. He KNEW that he was one step ahead of everyone in Kings Landing. He KNEW he could beat anyone who was thrown at him. And he KNEW that if he got the chance to fight the Mountain, he could not only win, but he could beat a confession out of him.
His weakness: Hubris.
Sure, the Mountain was on his back, spear in belly. Sure, any other man would've keeled over from that. But he forgot that he was dealing with a man called 'The Mountain Who Rides'. A spear to the belly was not going to kill him.
Plus, yes the Mountain was in heavy armor, but I don't think we haven't seen him in that armor yet. He literally lives in it. It's like a second skin to him at this point. If it wasn't, his size and strength wouldn't count for anything. A smaller, quicker man would've killed him by now. So we know he has to be unnaturally quick too.
Oberyn, as much as I loved him as a character, was too much in his own fantasy to even consider defeat.
People say that GRRM likes to kill off characters for fun, or for shock value. I don't think that's the case though. I mean, he may have a nice little chuckle here and there, and there may be plot defining deaths that he KNOWS he has to write in from the start, but I think he's a bit more of a naturalistic writer. I think he kills people only if they do it 'to themselves', or get really unlucky. Sure, he kills off a lot of like-able characters, but that's because we've been conditioned to think that a character who is heroic, and/or good, is invincible.
In actuality, if you read (or watch) most popular series out there, the hero is getting constantly into a bad situation, mostly because they put themselves there. Look at Harry Potter. He should have died many times over. In a more 'realistic' (as realistic as HP can be) book, he wouldn't have lasted three books. GRRM doesn't write a death for shock value, he writes it because in a more real world setting, that person would die.
Anyways, that's my take on the situation. RIP Red Viper. RIP.