1. Use AWS
2. Use Postgres
3. Use AWS
4. Use Cassandra
5. Use python, so later you can write C when shit needs to go super fast
That's what I got.
Those are some of the important lessons, although use (postgres|cassandra) are really too prescriptive. More like "use the right tool or tools for the job".
Also, use consistent key hashing where appropriate is another important less that I should emphasize more.
And "build for 3" is another important lesson. It makes scaling much easier.