While distaste for Tom Cruise and his growing level of (for lack of a better term) wackiness is a good and valid reason for avoiding this movie, and will probably keep me away from other movies that he stars in or produces. But my reason for staying away from MI:III is much more fundamental: I am still mad at how the movies, or at least the first movie, was a complete betrayal of the concept of the TV series they are supposedly based on. And no I'm not (just) talking about the name of the character in the first movie who turned out to the traitor.
In the TV series -- both the original from the 1960s and 1970s and the update in the late 80s -- the heads of the IMF (both Brigs and Phelps) put together a team of experts, actors, engineers and others, who were not necessarily part of the government to pull off tasks -- usually elaborate cons -- that the government couldn't officially do. Due to the reality of TV, the team was almost always the same people -- at least in a given TV season -- but the idea was that anyone could be used.
As I recall, in the first movie everyone was part of a team working for the CIA, and most of them got killed until we were just left with Tom Cruise playing, more or less, an American James Bond without the British charm.
Now, I'm also a fan of Bond -- or at least classic Bond, and I enjoyed the recent ones I've seen -- but Mission Impossible is not James Bond. The Mission Impossible TV series probably had more in common with the Con Game genre (Maverick, occasional episodes of The Rockford Files or Remington Steel, and The Sting) than the Spy genre (Bond, The Man from UNCLE, Get Smart, etc.). But the writers and producers of the movies appear to have ignored that completely, and just created another super-spy movie series with an over hyped popular actor in the lead.
Now, if they fired everyone involved and made a clean start with a new series of movies truly based on the TV show (one of two dramatic shows I enjoyed as a child and still enjoy when I catch -- the other being a space opera that shares a common studio and a common actor) I'd consider spending my money on it. But, if they continue to make super-spy movies, I'll continue to stay away.