Non-fatal casualties. In Star Wars, no one ever suffers any serious injury in battle. No arms or legs blown off. Nobody laying on the ground screaming in agony with his guts flayed open. Every combatant is either unscathed, knocked unconscious without injury, or killed instantly. Now, I recognize that Star Wars is not a gorefest, but it would be nice to see soldiers suffering some kind of injury, as opposed to either being untouched or dead.
Clone troopers (who just acted like better-equipped droids in AOTC) demonstrating that they are human by doing characteristically human things such as flinching in the face of enemy fire, looking for cover, hitting the dirt when enemy fire comes in, dragging wounded comrades to safety, etc. There is a limit to how sterile a war should be, even in a popcorn movie.
Boba Fett's revenge. Actor Samuel Jackson has said that he wants his character Mace Windu to die a memorable death in Episode 3. I think that his death should come at the hands of Boba Fett. Picture this: Mace Windu fights off a horde of attackers. He is victorious, but he senses danger and looks skyward, only to see Slave 1 racing towards him. Boba Fett, who can barely reach the controls, looses a seismic charge and then veers off. Windu's expression changes to one of fatalistic resignation as the seismic charge hurtles to ground, and the next thing you see is Slave 1 flying away from the titanic blast. Inside the ship, Boba whispers "got him, Dad". And the circle is closed.
The mother of all space battles. It's the last Star Wars movie that will ever be made, and I want to see some war among the stars! I want to see warships filling the screen with destruction, and fighters swarming like bees wherever you look.
Darth Vader. I will be very disappointed if we don't see Anakin in the full Darth Vader armour at some point, acting the way we remember him from ANH. Not just bratty, or selfish, or vengeful, or even psychotic, but cold, ruthless, methodical, and evil.
The mighty Executor, along with star destroyers, TIE fighters, and other vessels which will stylistically join the prequels to the original trilogy.
Orbital bombardment. The ability was mentioned but stymied in TESB by the defensive shield, and it was ruled out for unknown reasons in AOTC. I find this frustrating, and I want to see it! However, I have a feeling we won't.
The command apparatus of the Imperial Navy. By that, I mean the black-suited officer corps, the green-suited admirals, and the Moffs, including Grand Moff Tarkin.
PS. Many readers have written in to add that they would also like to see Natalie Portman's tits. However, while this would indeed be pleasing to the eye, I don't think it will happen :)