Hit level 55, you get 4 hardmode revamps (Cad, Mando, Athiss, Hammer) that are actually quite a bit different (bonus bosses, new mechanics, more trash) than the leveling versions.
Actually, the mechanics for the bosses are largely the same (I was quite disappointed by this). The only major differences are that most of the mechanics were made less forgiving (which is what happens when stuff is turned into an HM). The only bosses that actually got a mechanic *added* were Xander and the droid in Cademimu (the droid has to be trapped in the circle rather than just kited and nuked down), General Ortol in Cademimu (the magnetic shackles were added), and Vorgan the Volcano (the shield was added). Every other boss mechanic was present in the story mode version, though most people simply ignored a lot of them while leveling because you, honestly, could.