Startup Principle: Be a One-Trick Pony
This is CRITICAL to a startup/company in the 0 -> 1 and 1 -> 100 stages.
I want to start by acknowledging that startups have to deal with the inevitable, difficult, trade-off between staying focused vs. being opportunistic. There are enough cos. doing either, and succeeding. Uber is a great example of a company solving a specific, focused problem. So is AirBnB. Conversely, the Rocket Internet group operates as an opportunistic player in the eCommerce space.
However, most companies aren't able to successfully emulate Rocket Internet's rather nique operating model.
However, Uber or AirBnB's focus - or one-trick-ponyism - is definitely a trait other startups can emulate. Most companies that build amazing products, scale fast, build great traction, and have (relatively) uniformly good promoter scores from their customers, do so on the basis of focus.
Look around you: Google, Facebook, IKEA, Apple, Nike etc.
The advise is simple: Do one thing, and do it better than anyone else.