Personally, I'm fine with Megatron as a tank. I know it might not becool with G1 purists, but I always thought Megatron's transformation into a gun was cheezy. First, he inexplicably shrinks, and then you're left with what? A gun. Since almost all the tranformers already have guns, this transformation is especially useless, and it also makes him subservient to whoever is "using" him (so that's why he keeps Starscream around - FIRE ME, BOY!) Apparently he was able to fire himself from mid-air or whatever, but honestly he was a cool toy but a terrible transformer.
Now, Camaro Bumblebee, yuck! That's stupid. The idea of changing an old Camaro one into a new one halfway through the movie is just the strawberry jam on this rancid hotdog. It really exposes the corny advertising ploy for what it is. I could probably live with it if they just started out with the Camaro concept, though it would still be wrong. I like Donutt's idea of having a badgeless Beetle-like car, but I guess that won't fly in this day and age of corporate-sponsored everything.
Possible changes to the storyline don't bother me so much. The series has been reimagined so many times now, I'll forgive an inaccurate retelling of the story if the acting and writing are decent. That and the effects, obviously.