Spartacus is about having the maturity level of Channel 5 when it comes to sex and violence, ie showing as much of it as they can simply because they can show it, with only John Hannah's exclamations anywhere near actual entertainment...
GoT is very much a massive drama-llama, albeit a very engaging one.
Vikings aims to be far more subtle in its approach, I think. It doesn't suddenly plunge you into anything or try for the shock/wow factor, but still manages to drag you into it. For me, Vikings succeeds where Spartacus, True Blood and so many other similar shows just fail to deliver.