I know it's limited by the user's willpower and imagination, but is there anything the rings truly can't create?
As far as physical constructs go, it would appear that the ring is limited only by the wielder's imagination and willpower. The one exception to this would appear to be life. A Green Lantern can create something closely approximating a living being (as seen in Green Lantern Vol. 3 #48), but actual life is beyond the ability of a ring to create. It should be noted, however, that Hal Jordan thought the Central Battery on Oa contained enough power to do so:
That being said, the Green Lantern ring is capable of creating anything from microscopic nanites to entire planets. Even the color of said constructs is not limited to green. In one case (Green Lantern: Ganthet's Tale), Hal Jordan was able to shift the color of his ring's construct to yellow:
Green Lanterns have also been able to create semi-permanent constructs that last long after they leave, as well as influence life on a genetic level (both seen during the Rann-Thanagar War). And, of course, any Green Lantern with the sufficient knowledge can create additional Green Lantern rings as needed, although this is rarely done.