You have to run Hammer directly from the base app bin directory, no more using the SDK launcher.
Steam\steamapps\common\Source SDK Base 2013 Multiplayer\bin\hammer.exe
Probably the most awesome thing so far:
Extra decimal places in the texture scaling (left 4 dead hammer had this), so you can make higher res textures and properly texture the maps (1024x1024 texture on a 128 high wall, for example...giving
8 pixels per unit instead of the typical 4 in most source games/mods).
Ooooohhhh, ahhhhhh
* basically, the old hammer that mods were allowed to use only had 2 decimal places, so for example, trying to scale a 1024 texture to 0.125 on a 128 length brush would just break and it wouldn't scale properly *
