This might just be because we're in 540+ gear, but normal mode SoO also feels rather unrewarding in its difficulty. I think it is a good thing to have some bosses easier than the others, it helps to keep morale up and rewards you for killing a hard boss. I think we'll have to wait another few weeks before saying whether or not the instance is too easy, but we killed Lei Shen normal in the week before it was released on LFR. We'll likely be at him on Sunday; a week or two before the second of four LFR wings releases. So our progress in SoO has definitely been quicker than in ToT (which we started with around 505 gear).