I'm sorry, The Rock is not an "accomplished" actor, he's a wrestler trying his hand at being an actor. And I HAVE seen several of his movies. The Scorpion King was... okay... I guess, but I didn't have any real expectations about that film. His best role, that I've seen, is in Walking Tall. In that movie, he wasn't a bad actor, yet he wasn't a good one, either. The Rundown was... okay... but not great. Seann William Scott is what made that movie good, not The Rock. I haven't seen Be Cool yet, but then again I haven't seen Get Shorty either, which I would want to watch first.
So, your thought that I haven't seen his performanaces is unfounded. I have and The Rock might become a good actor, someday, with more work and training. However, I DON"T think he'd make a GOOD Conan. He isn't the right type of personality, IMO. It is like barsoomcore said, he doesn't BROOD well, which is vital to the Conan character.
Now, as for Vin Diesel, his performances have ranged from very good (Pitch Black) to very bad (The Pacifier), and I'm not sure if he would or would not make a good Conan. If he pulled off one of his good performances then maybe. Regardless, at least Diesel is an actor first and foremost. He may not being doing Shakespeare any time soon, but he could do the BROODING warrior type that is required for Conan. Would it be a great performanace, I don't know. We'd have to wait and see.
But I'd choose him over Triple H anyday, and would prefer him over The Rock, as well, for the reasons mentioned above. In reality, he doesn't look enough like REH's Conan. But then again neither did Arnie.
Anyway, I'll just agree to disagree with you about it.