It seems miraculous that YouTube has so far avoided a major lawsuit - that's mainly thanks to getting the major media companies involved early, and offering them a stake in the company to keep them off YouTube's back. That said, legal action still seems possible, and the latest group to threaten a lawsuit is the independent music labels.
The independent labels are aggrieved, says Billboard, that they haven't been offered sweet deals like the majors and that royalty negotiations with YouTube haven't gone well. The new conglomerate label Merlin, which aims to represent the indies, may also get involved in defending the artists, it seems.
But frankly, this is the oldest negotiating tactic in the book: if YouTube doesn't give you enough money, threaten to sue until it does. The vast majority of the time, the two parties come to a revenue sharing agreement a few weeks or months later. Only when things turn really sour is any action taken.