Also not really a heuristic, but "differentiation is easy," as encoded in the following two sub-heuristics:
- Differentiation is just repeated application of the product and chain rules, and
- Most functions are differentiable most of the time.
Edit: Someone doesn't seem to like this answer, so I'll expand. Students who leave calculus with this impression enter analysis with a disadvantage: differentiation is not a property that "most" functions have in any reasonable sense, not even continuous ones, and to compute the derivative of a function that isn't given as a sum of compositions of "elementary" functions requires an entirely different mindset than the one that values the product and chain rule.