These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The song that Harry Heck sings to Frank: "In Time".
Better on DVD: The "Extended Cut" release; several scenes are added in that actually improve upon the film's theatrical cut and flesh out a few plot holes - most specifically, scenes with Frank's former partner, who is revealed to have sold Frank out to Howard Saint, which leads to the Castle family being massacred. (The "Extended Cut" of the film is also the one shown when the film plays on the FX network.)
Ensemble Darkhorse: Harry Heck. A Johnny Cash-esque, Affably Evil murderer who sings the tune to Frank Castle's life. Then he dies one chase later. The character made quite an impression on audiences however, and is often mentioned as one of the most memorable scenes of the movie. Keep in mind Harry Heck was nothing but a named Mook in the comics who appears and dies in one page.
Harsher in Hindsight: John Travolta's character kills Frank's family as payback for the death of his son. Five years later Travolta's son died.