I've heard the jump from Story to Hard is quite big, since it's tuned for full a full Rakata raid minimum.
I've not personally done HM Toth and Zorn (first attempt will be tomorrow
), but we managed to beat Kephess in like 4 or 5 tries.
It was all down to the mechanics really, once we got to grips with what was happening, how to deal with the different adds etc is was pretty easy.
Phase One on Story for Kephess is definitely the hardest, once he actually drops down it's pretty easy as long as your tanks know when to switch (or how to kite if you're kiting).