Is veganism an act or an omission?
One of the more troublesome aspects of this distinction is how you can decide which things are acts and which are omissions. Discussions tend to center on cases where there's consensus about how to describe things -- e.g. pushing a boulder down a hill onto someone is an act, but failing to move a second boulder to block the path of a naturally falling boulder is an omisison. The great variety of explanations that philosophers have come up with to justify the act-omission distinction only complicate the process of boundary-drawing.
One example that occured to me today is veganism. Let's assume for the remainder of the argument that we have some good reason to see not eating animals as better than eating them, and that we're talking about people who are medically and socio-economically able to be vegan without extreme hardship, and that "veganism" in this argument is referring to a dietary choice, not any additional activism for the cause. Is veganism then a praiseworthy act, or merely an omission of animal cruelty? Conversely, is omnivory merely an omission of a higher moral calling, or is it an active violation against animals?
The simplest case for the "act" side is that veganism seems to involve a great deal of conscious effort. People who never put any thought into the idea that there might be some moral issue about eating animals slide easily into omnivory. To be a vegan requires looking up recipes and nutritional information, reading food labels, dealing with temptation, dealing with unsympathetic friends and family, etc. But this effortfulness is not intrinsic to veganism -- rather, it's a product of the social situation that is set up on the assumption that everyone eats meat. In a vegan society, it would be effortless to remain vegan but would require effort to eat meat. Following this line to its logical conclusion, "acts" end up defined as "things that go against prevailing custom" and "omissions" as "conformity." It seems odd that a distinction so allegedly significant to moral action would be so deeply culturally relativistic and conservative.
The most obvious case for the "omission side" notes that veganism is most commonly defined by a negative fact -- vegans are people who don't eat animals, that is, people who decline to commit a wrong. After all, the slogan is "meat is murder," not "meat is manslaughter." However, it would be easily enough to reframe the definition of veganism in positive terms -- "vegans eat only plants and minerals." This suggests that negative facts, which often play such an important role in the act-omission distinction, may often be a matter of framing and linguistic convenience rather than a metaphysical truth about the event being described.