Tooth wear in relation to food, has mostly to do with the molars, like Diane R mentioned. Incisors are primarily used for cutting and snipping things, then with the tongue, move it back to the molars for grinding. Some foods can have a minor wearing effect on the incisors though. Like apple branches, where the rabbit uses their incisors to strip the bark, and other small branches that a rabbit will use the incisors to cut into smaller lengths to chew, and hay cubes a rabbit also uses their incisors to strip small bits off for chewing. But the main component of incisor wear has to do with proper alignment, so the top and bottom teeth properly wear against each other and the peg teeth.
For the molars, it's a combination of chewing motion and the amount of abrasiveness in the food that's being eaten. Pellets do have hay in them, so some amount of wear will occur from the food and the molars contacting each other, but not as much as occurs with long stem hay. Pellets are chewed in an up and down motion resulting in very little lateral side to side tooth contact, which is what helps the molars wear against each other and prevent sharp points from developing. With long stem hay it's a more circular chewing motion, resulting in good lateral side to side contact of the molar crowns, creating more surface contact and even wearing of the crown.
The other essential component is the silica in vegetation that provides the abrasiveness needed for optimum tooth wear of the molar crowns. Silica is like sandpaper, 'sanding' down the surface of the molar crowns as the rabbit chews on the hay/grass/vegetation in a circular motion and side to side laterally. Some vegetation doesn't have much silica content and so will create less tooth wear, some has a lot, some grasses have more than others and will help even more to keep the teeth worn down well. So for good tooth wear it takes both the abrasiveness of grasses and the chewing motion required to break down the long strands.
Here are a few links that cover some of this info.
http://mohrs.org/hay-dental-health/
http://mohrs.org/rabbit-teeth-how-they-work/
silica in hay and how it applies to rabbits