Viggo does menace well in my opinion, though he also does vulnerability quite well.
I already mentioned Eastern Promises and A History of Violence - really enjoyed his characters in both those films, and in both he's effectively a soft-centred man who has become addicted to, and inured to, violence through his traumatic experiences.
I can't see him playing an "everyman" or a comedy role terribly well - the sort of thing that Tom Hanks might take on would be too hard an ask (I believe). Though on the other hand he might be able to manage the sort of farcical comedy roles to which Clooney and Duchovny tend to gravitate.
As a leading man in LotR or Hidalgo he's just average, although I do hear he goes down well with the opposite sex.
