Some people are reporting that the main menu is broken, like clicking Options just results in all buttons disappearing, or the Optimization button not doing anything, etc. These following steps seem to alleviate the problem:
Delete these folders:
C:\Program Files (x86)\Steam\steamapps\common\Natural Selection 2\core\lua
C:\Program Files (x86)\Steam\steamapps\common\Natural Selection 2\ns2\lua
All paths relative to your steam directory of course. And then after deleting, go into Steam and re-verify your game cache (to redownload those particular files, only a few MB).
Delete these folders:
C:\Program Files (x86)\Steam\steamapps\common\Natural Selection 2\core\lua
C:\Program Files (x86)\Steam\steamapps\common\Natural Selection 2\ns2\lua
All paths relative to your steam directory of course. And then after deleting, go into Steam and re-verify your game cache (to redownload those particular files, only a few MB).