This Blender release has a lot of new tools and goodies.  I’m most excited about these for building in virtual worlds:

• Drawing freehand curves!
• New bevel options
• Region select
• Carver – Let’s you use shapes to carve away at an object. Common tool in other 3D programs and really welcome.
• Archimesh – make walls, door, and windows easily. Will be interesting to see the poly count on complex windows

The usual cautions about polycounts apply on all of these tools. Before you bring a carved up object inworld, take a look at the geometry and poly count. Then reduce it. I understand there are some improvements in the decimate tool.

Here’s a nice overview of new 2.78 features by CG Masters.

See the release notes for the full story on changes in Blender 2.78