A minor quibble, but I thought Viggo was a bit too good-looking to be Aragorn... a character who was supposed to look fairly rough and "foul-looking", and unbelievable as a "king". The designer stubble just wasn't a convincing disguise, IMO. Especially with the cheap trick of having it become a full beard for the crowning.

But, as I said, a minor nit-picky quibble. I couldn't find fault with his acting at all, but then, I'm no actor.