I felt the same, wish there was more substance to the fight. A lot of the spear twirling didn't do much for me. I kind of know how the story goes but it still annoyed me that if Oberryn is really trying to avenge his sister's rapey death, you stay out of arms reach till Greggor's finally dead right? It's in the script as they say.
Eh, I'm not so sure I can agree that it was 'annoying.' I see it as entirely believable, given the premise, the individuals and the stakes involved. Especially in a world where people that are skilled with weapons and fighting are practically gods among men.
Oberyn was a very fiery person who had nearly 15 years of rage, a stain on family honor when family names and honor are a really big deal, and a burning desire for revenge brewing in his heart. More than just killing Gregor, he wanted Gregor to point to Tywin Lannister with his dying breath. He was punishing Gregor for a lot of that fight, while taking a few hits himself. He didn't just want Gregor dead. Gregor was the weapon, but Tywin was the hand that wielded that weapon. He wanted the whole world, all the important people that mattered, to see that Tywin was the architect of Ellia's death. Remember, he didn't say he hated the Clegganes. He said he hated the Lannisters, and that was the reason why. I'm sure he despised Cleggane as well, but there was a bigger endgame here and Cleggane was just one part of it.
I think that no matter how much you say 'Everyone knows this or that,', you have to carry that type of hatred with you for the same amount of time, and let it gnaw at your soul and at your guts, and live in a world of swords and spears, with all those intrigues and politics, in order to even begin to think about simply efficiently killing someone under those circumstances.
I have absolutely no issue with Oberyn talking up a storm while taking Gregor apart. Yes, he made a mistake, but passionate people often did and often do. I think this is an instance where Martin thought about the motivations of the characters and did it right, given the world they live in.