So one of the things I'm most excited about with H16 is the new HAPI 3.0 for the Houdini Engine. What kind of improvements / additions can we expect? I haven't been able to find any details about what's new.

For a comprehensive list of changes (which will give you an idea of what's new) can be found in the Houdini Engine 3.0 docs here: http://www.sidefx.com/docs/hengine/_h_a_p_i__migration.html [sidefx.com]

But basically, the biggest change is that the concept of the “asset” has been replaced entirely by “nodes”. Everything is now a node and all APIs that used to take an asset, object, and geo id now just take a node id. It simplifies things a lot and also gives you more power over the underlying Houdini node network.

Among many small improvements, HAPI 3.0 also supports some of the new features in Houdini 16 like HeightFields (Terrain) and VOP-based materials.