Every time you use your Car/Jet mode's superior speed to get close to a Mook, shift forms and immediately launch a melee attack without stopping.
Pulling off a successful ram in heavily armored Truck mode.
Using your tank mode's turning ability to outmaneuver a car or truck and finish them off while they try to figure out what you just did.
When Megatron is being chased by Omega Supreme, he is very reluctant to run away from the special super-robot trying to kill him (and who could kill him with a single direct hit), actually daring him several times to come and get him.
Two words: Atmospheric freefalling. To further elaborate, after you force Trypticon back into his natural state, he begins freefalling down to Cybertron from space and you keep shooting him as he does so! The best part of it all? It's entirely playable.
The first time you play the Autobot campaign and you hear Optimus Prime say "Transform and Roll Out!" No matter who you play as, you'll be pumped to pressed the button to transform.
Brawl taking out an entire squad of Autobot troops who were dogpiling him during the assault on the archives. Since he's the only one of his team to appear in the main story, it's a nice demonstration of why they're called the Combaticons.
How does Bumblebee make his introduction to the game? By driving past Decepticons firing at him and up a ramp, transforms to robot mode as he flies through the air and shoots down two of them, then transforms and lands safely on his wheels. Doubles as a Crowning Moment of Awesome for Optimus Prime, who comes to his rescue.