Quick answer: just don't prematurely screw it up. Some features and business ideas just don't scale. The quickest way to check is to imagine you have everythi

Quick answer: just don't prematurely screw it up. Some features and business ideas just don't scale. The quickest way to check is to imagine you have everything in a big dumb (semi-normalized, but not fantastic) database. Eyeball the queries you're going to use. Then figure out how much work the database has to do to execute those queries. Also consider read/write rates and contention. It's back-of-the-envelope, but it's pretty good.