some other differences from the cage and ring.....
-When fighting in the ring you can get "stood up" when you "fall out" of the ring. It can actually give a fighter a tiny breather and in some cases actually turn the tide of the fight. Cro Crop himself said this was a huge difference from the cage and the ring. No "breathers" in the octagon...I have never seen a guy "fall out" of the octagon before:142smilie
-Another aspect is in the octagon it is more difficult to "corner a guy off". Meaning if your in a ring you can cut a guy off (he trys to run away) much easier then in the octagon:SIB
Yes it may not be a HUGE difference fighting in the ring as opposed to the octagon, but I assure you of this, Rau will have a little bit of a wake up call when he steps into the octagon.
I've never seen anyone fall out of the UFC octagon, but Tank Abott almost threw Cal Worsham over the top :142smilie . I have however seen non-UFC events (for example, Strikeforce: Bobby Soutworth vs. James Irvin) where the cage door was not closed properly and both guys blew through the door on a takedown.
Rua will certainly require some adjustment, but he has faught in a cage before although the flooring wasn't fitted properly and you could see it bunch up, it was rediculous. It was several years ago at an IFC tournament, which interestingly enough Forrest participated in. Rua lost to Sobral by sub and Forrest got koed by Jeremy Horn with a head kick. Sobral beat Jeremy in the final.