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.
Matrix in Hammer has a debuff that adds +500% turret damage when he moves from platform to platform, turning a minor inconvenience in story mode into a guaranteed wipe.
Tunneling droid in hammer station you generally didn't _have_ a cleanse yet to clear the tank stacks so it is a different thing for healing.
The others are significantly tighter and you have to do the mechanics right, something that makes the fights totally different from the story mode tank and spanks.
And then theres the trash revamps, trash is no joke until your in 69's...and even then if you're careless some of the pulls will decimate you, it's a lot less forgiving than it is on story mode. There are also more packs added into the FPs, go into Mando Raiders right now and look at the entry hall.