Most of the seasons have proven entertaining (while the third disappointed slightly, the fifth achieved an epic height) until the sixth season, which (despite a promising opening quartet of episodes) very nearly drove the show into the ground.
I always thought season 4 was 24 big opportunity to renew itself. We had Jack leaving CTU and working for Secretary Heller, so they could've easily moved the setting to another US city. They could also have introduced a bunch of new characters (which they did, but in the second half they just took back the old one's). And they could have made the threat not something big with catastrophic consequences (again, they did have the Heller-abduction, but they went back to the formula in the second half).