"Arthur's Bane" is Mordred, who is supposed to return in Series 5. As Killgarrah said, Arthur would find it's end as the resoult of a "Final Alliance between Mordred and Morgana", just like in the original Arthurian legend.
It's Arthur himself. His difficulties and duty as a King of Camelot will be more difficult after time. And theese difficulties will come between him and what he truly stands for. For example his respect for his father rules, that he either wishes to change or discard. (the magical use for instanse) Arthur's Bane is what I think is obvious in the case is himself.
Uther. His ghost or memory will haunt Arthur and make things difficult for him as the king. Much later, Arthur will confront Uther and there will be a resolution. For instance, suppose Arthur discovers Merlin's secret. He's unsure what to do and Uther appears telling Arthur what his duties are. Arthur's now torn over what Merlin's fate will be.
I suppose it refers to Mordred, but I think somehow it will be more complicated than that. I don't think Mordred will be a complete villain. Possibly he will be torn between his loyalties to Morgana and Arthur. It seems like Mordred would be more of a bane to Merlin than Arthur. I look forward to finding out.
Well seeing as bane is old English for death or destruction or what causes it I imagine that this would be mordred as in the original stories. Kilgarrah told Merlin in the early episodes that mordreds destiny was to kill Arthur therefore mordred is Arthur's bane :)