[Tristan Lall]

There are several tradeoffs involved in designing heatsink fins, but the main one is providing enough conduction to supply the fin with heat, and enough convection to remove the heat from the fin. (Radiation is often negligible, unless you're in a non-convective environment, like space.)

Convection is affected by fin spacing and airflow. In general, active cooling with a fan can use more closely spaced fins. Passive cooling using natural convection tends to employ widely spaced fins. The most efficient fins in natural convection are often cylindrical or conical, but those are fairly difficult to make, and so straight fins are substituted instead.

Additionally, conduction is affected by the thermal resistance over the material interfaces. If the heat has to cross a lot of material boundaries to get to where it's convected away, the heatsink will tend to be less efficient.

You'll also find that some areas of a heatsink are less efficient than others, relative to the mass of the material used. The squarish blocks on the corner of your design come immediately to mind: they're going to be relatively cold (because they're far from the heat source and exposed to the air), and thus have a small temperature difference with the surroundings, which means less heat flow.