The reason is simple: his presence sucks the energy from the screen. It's not precisely that he's a bad actor. He is perfectly convincing, in a certain glum way, as a rogue cyborg, a crippled-marine-turned-Na'Vi-hotshot, or a demigod with a grudge. The problem, instead, is that his performances are almost stubbornly generic. He resists anything -- any flourish or exercise of actorly prerogative -- that would give any of his characters a shred of life or personality. They are like zombies carrying out the screenwriter's instructions. He gets a ton of screentime in the films discussed above, and while I can't recall a specific moment in his performances that strikes a false note, I can't recall a single memorable or interesting one either.
The issue, then, is a little more nuanced than "Sam Worthington ruins movies." He doesn't. The issue is opportunity cost: if this trend continues, Worthington will continue to take starring roles away from actors who might affirmatively bring something to them. I suspect Clash of the Titans, which I think is thus far the worst studio film of 2010, was beyond salvaging, but the prospect of Christian Bale taking on the part of Perseus is nonetheless tantalizing. Or what about -- hell, I don't know -- Will Smith as Jake Sully. Or Jake Gyllenhaal. Or Ben Foster. Or take your pick. I bet you can come up with any number of actors who would have been more interesting in the role than Worthington was.
There may be depths to the actor that I didn't have a chance to glimpse in these intense roles. I haven't seen him as a contemporary Macbeth, or the slew of indie films he made in the first half of the last decade. His next two films, John Madden's The Debt, about Mossad agents pursuing a Nazi war criminal, and the romantic drama Last Night appear to be of a different sort. I just hope that the unprecedented success of Avatar and likely success of Clash of the Titans don't make him a fixture as an action hero.