We now have Vin Diesel and Groot and Bradley Cooper as Rocket Raccoon. I would rather have a seasoned British voice actor do Rocket but we will see what happens.
Also, James Gunn said they are already over halfway through shooting. It's only been a couple months so they are flying through it.
It's an ambitious overall story based on the characters involved. Outside of the Guardians, you have the Nova Corps and the multiple villains of Ronan the Accuser, Nebula and Thanos in the background. Also, the Collector fits somewhere in there.