Forget about talk about disappointing writing and McG's choppy direction, there's only one reason why Terminator Salvation failed at the box office, and his name is Barack Obama. Don't believe me? I have a chart.
Look at who was President of the United States when the last Terminator movies were released (Or, for that matter, when Terminator: The Sarah Connor Chronicles got launched on Fox). Notice a trend?
Apparently, Republican administrations mean good things for Terminators (Insert your own robotic overlord joke here)... Weirdly enough, the inverse can be shown for Star Trek, which got its start (and a couple of rebirths) under Democrats, as well as seeing its most popular period while Clinton was in power... before dying an awkward death under a Republican regime (As long as you ignore the fact that The Next Generation got its start while Ronald Reagan was president. I get around that by pointing out that the show wasn't any good until the third season, and then saying that it's the exception that proves the rule, before looking at the ground, embarrassed).
Coincidence, or something more sinister? You be the judge, but maybe there's a case to be made for the connection between audiences feeling more in tune with the end of the world when there's a right wing President in office, compared with the more utopian values a liberal president inspires in us. Causation or correlation? You be the judge.
(* - This may not be entirely true.)