Data driven decision making. Everyone talks about it, knows its critical and says they're doing it. But when push comes to shove, and decisions need to be made, as many would have experienced, it's far from straight forward.
There has been much said and written about good and bad ideas in the startup world. Most notable and influential to me being Paul Graham’s essay around its counter intuitiveness  and Peter Thiel’s Venn diagram of ‘looks like a bad idea’ and ‘is a good idea'.