G1 Prime has the following traits:
- As mentioned, a softie. (Nothing wrong with that.)
- Merciful. (Nothing wrong with that.)
- More the Savior figure, than Movie Optimus.
- Showed kindness and patience.
- Tried to keep humans safe from conflict.
Movie Prime has the following traits:
- Not a softie. (Capable of killing, has no reservations about it.)
- Is merciful, but never given the chance to show it. (Megatron did NOT offer him a real truce in DOTM.)
- Not the savior figure, though they tried to work it in in ROTF.
- Impatient, at times. Not exactly warm and fuzzy.
- Took the battle right to the streets, people or no. Let the Decepticons run rampant in Chicago to prove a point. To show the humans they were wrong. (G1, the humans would've listened to Optimus and all's well that ends well.)
That's the problem. G1 Optimus is an idealist. He's a hero everyone can look up to.
Movie Optimus is a realist. He's a hero, but at great personal cost. He's the hero that makes you turn your head, while he cuts the guy's throat. He does what you'd actually need to do in a war. Unlike G1 Optimus, who was given an out due to the target audience. Movie Optimus is far more realistic and shows the scars of war, inside and out.
He'd kill because he has to. He wouldn't hesitate. G1 would hesitate. That doesn't make him less powerful, just doesn't allow him to be a formidable foe.
It's like Bruce Willis said in Live Free Or Die Hard: http://www.imdb.com/title/tt0337978/quotes?qt=qt0475112
John McClane: You know what you get for being a hero? Nothin'. You get shot at. You get a little pat on the back, blah, blah, blah, attaboy. You get divorced. Your wife can't remember your last name. Your kids don't want to talk to you. You get to eat a lot of meals by yourself. Trust me, kid, nobody wants to be that guy.
And that is the kind of guy Movie Optimus is.