While this latest from Dreamworks does a relatively decent job of delivering three of the four throughlines needed for a complete story, it is the absence of the fourth that scars this story.
Roxy, as Impact Character, is a complete zero.
Sure, she tells Megamind that he can be a force for good, but you don’t manufacture change in someone by telling them they have to change. Most will reply with a kind how-do-you-do and send you on your way. No, the way to have meaningful impact on the Main Character is to provide an alternative perspective on issues that he or she is dealing with. This perspective should be similar enough to the Main Character as to warrant some kind of recognition, but different enough that it resonates internally, providing energy for a meaningful shift into new territory (also known as Acts). This is how effective character development proceeds.
In Toy Story Woody had Buzz’s bravado and way of thinking to deal with. Marlin had Nemo’s comfortability with his physical affliction (small fin) to contend with in Finding Nemo. In Megamind it should have been Roxy’s cynical attitude towards the real Metroman — the Metroman only she knew. Her recollections of the supposed hero as someone who only cared about himself would have clashed with Megamind’s assumptions about his nemesis (and would have supported the surprise twist tacked on at the end). The revelation of a lifetime of believing what amounted to lies would have had a direct impact on Megamind’s issues of destiny and the path he was to take. His eventual change, therefore, would have carried some meaning with it.
Also sorely lacking is any semblance of a Story Goal, a glaring hole that can be felt the moment Metroman meets his early demise. Any sort of narrative drive that existed previously is lost as Megamind the Protagonist wallows in self-pity. Again, the natural solution would have been to have Megamind pursue a place for himself within Metrocity, a goal that would have begun with him imagining a new city teeming with evil and dastardly deeds and ending with a new concept of himself as the Hero. As it stands the latter part is there in the final film, it is the beginning that is painfully missing.