What does it take to run a web app with 5K – 10K users?

A few years ago I was inspired by this artice: http://viniciusvacanti.com/2010/11/01/6-things-you-need-to-learn-to-build-your-own-prototype/. Now I am able to build my own Rails apps and run them in production. I am now wondering what it takes to launch and successfully run a website with 5-10K users? All the articles I have found doing a web search were vary basic.