The main criticism I've found is that the ultra-violent second half doesn't mesh with the gentler, character-driven first half. Personally, I think you need the first half to understand why Denzel Washington's character, John Creasy, takes such brutally violent actions later on. The two halves compliment each other by providing an understandable motive for why Creasy does what he does.
However, I've gotten ahead of myself.
For those of you who don't know, Man On Fire (2004) is about John Creasy, a former CIA operative who is troubled by his violent past. He is directionless, emotionally distant, and an alcoholic. With the help of an old friend (Christopher Walken), he gets a job bodyguarding Pita Ramos, the young daughter of a wealthy Mexican business man.
The daughter, as played by Dakota Fanning, is both precocious and determined to break through Creasy's gruff exterior. She eventually does, and Creasy slowly allows himself to care for another human being and be cared for in return.
It goes without saying that Pita gets kidnapped. And that event is what sets Creasy off on his violent rampage, partly for revenge and partly to find out who is behind the kidnapping.
Does it get pretty violent? Yes, the film certainly does earn its "R" rating. But, I never did feel the disconnect that so many critics felt the need to point out. That is because, for me, the real power in this piece is the relationship that builds between Creasy and Pita. A father/daughter, mentor/mentee connection forms between the two, and it is deftly and believably portrayed by Washington and Fanning. It's the belief in their deep affection that makes the second half palatable.