However, as we grew, scaling our existing features to accomodate more traffic turned out to be much less of an issue than adding new features.
In particular, making schema changes or adding indexes to a database with more than 10 - 20 million rows completely locks the database for hours at a time. Removing old indexes takes just as much time, and not removing them hurts performance because the database will continue to read and write to those unused blocks on every INSERT, pushing important blocks out of memory.