Jonesy hits the nail on the head with his comments. Never the best actor, Clint knew how to make the most of what he had and his brand of grizzled charisma made him an icon across two genres.
I enjoyed his earlier directorial work (Bird is very *********** in my opinion) and people forget that Unforgiven relaunched the Western again in the 90s (would we have Deadwood on the TV without it? Highly doubtful). Of late his movies seem to be a bit ponderous, going on for too long, or drifting towards saccharine sentimentality in the third act...
If you want proof of his star power you only need look at Gran Torino. In his late seventies he elevates what is a pretty sub-standard movie beyond the "made for TV" fodder the synopsis reads like and literally carries the film on his still ample shoulders. You'll forgive that he goes from serious racist to loveable old rogue in about the first twenty minutes and even delivers one of the slowest and most unconvincing punches outside of Eastenders. Hell, you'll even forgive him for singing at the end. He's just that good.