Speaking from my experience, which is limited within the bounds of a true BDSM relationship (my Sir is my first and only true dom), I would have to say there are differences between each level.
As has been mentioned this would range widely from relationship to relationship, person to person. What I may find dehumanizing, someone else may only see as objectification.
In my previous relationships (non D/s mind you) my boyfriends honestly ranged within the bounds of objectification, humiliation, and dehumanization (the latter of which I'm still making efforts to overcome).
Personally I enjoy being used for sex or a hole to fuck by my Sir. However as much as I'd enjoy having my brains fucked out, I do enjoy the fact that He puts them back in. When a relationship becomes solely based around objectification (unless this is a noncommital play relationship) I could see it leaning towards a dehumanization aspect.
When a submissive person (it's possible to be submissive in relationships without being D/s) is consistantly told they are worthless, degraded, humiliated, and considered only good for one thing it can have a negative affect on their outlook for themselves. This lends to the idea of breaking, in my personal opinion.
While I may be enticed by the idea of being called a slut in the throes of passion, being denied permission to cum until I beg properly, or tied up and left in a corner if I've been a cheeky monkey...there is still a boundary of being treated properly after the punishment or play time. I know my Sir loves me and cares for me, even if we're playing or I'm being punished, and afterwards I'll be told that I'm a good girl (if I was). I think it's that knowledge that changes the perspective from abuse for the sake of abuse to something more.