He's referring to the classic problem in a startup of distinguishing between things users want (vitamins) and those they really want (painkillers). It's critical to concentrate on the latter because you have so little margin for error.
I assume it means that some features are "good for you," while others are opiates; and possibly that this is a bad thing because opiates mask pain, and graphing pain is a crucial part of iterating a design, so the more opiates a design has, the more confounding factors there will be to evolving a good core product. (That's not to say that I agree that on-site search is an opiate, but an argument could be made for it.)
I don't have a better word to describe "painkillers", as he is trying to convey the difference between want and need. I want candy, but it's also not good for me. What word conveys that, and isn't harmful in some way?
