I like Hot Fuzz and Shaun of the Dead fine—Edgar Wright directed vehicles for his particular skill set.
But Simon Pegg is a skit man, he's not a situation comedy man. Mission Impossible is about very cool people being caught in very tricky situations, and that's how it generates humour.
The Star Trek reboot films aren't too great but he causes the same problem in those to some extent. To me it's a bit like when they put John Cleese in as Q in the Brosnan Bond movies. It's a category error, though Bond's a bit better able to incorporate it.
