[Ibrîniðilpathânezel]

I tend to think that Saruman's curse didn't work because he no longer had the power to back it up. As Gandalf had said earlier, he was a serpent who still had one tooth, the power of his voice. Other power he might have had in his embodied form as an Istar was no longer accessible to him with his expulsion from the order. Cursing the Shire was, I think, done for the purely petty purpose of frightening the Hobbits who had already seen the horrors done to the Shire at Saruman's behest. If it came to pass, it might have done so because the fear Saruman awakened in the Hobbits could make it a self-fulfilling prophecy. If they had believed he had the power to make the curse real, they might neglect the Shire, thinking that anything they tried was doomed to failure because of the "curse." Fortunately, the Hobbits were sensible enough to not let that happen.

As to the general efficacy of curses, I would think that some of the same factors that make oaths binding would also come into play. We know that part of what made the oath of Feanor so powerful and terrible was the fact that he called upon Eru and the Valar as witnesses to it. They are very real powers in Arda (heck, if TH is to be believed, some Vala has the job of enforcing the rules of riddle games). An oath is not the same as a curse, of course, so I doubt that Eru or the Valar would be inclined to make every curse called in their name come to pass. Some that are particularly justified, however, they might. When Melkor and Sauron retained enough power of their own, they were probably capable of making sure the curses they called down upon others actually happened. "Durin's beard," however, would not have the power to make a curse effective. "Mahal's beard," on the other hand, would invoke Aule, and if he thought the curse a valid one, he might act on it.