I assume you had a staging/test environment where you put the site up for some people to test (besides yourself and engineers)? It's important to use a variety of browsers and people to give things a good look over before going live. You have looked at it for too long so you'll gloss over it. Fresh eyes are so very important.

Did you hook up monitoring like NewRelic.com? They have a tool that will ping your site from a variety of locations. They also have insights as to where some bottlenecks might be.

Do you have backups for your database?

Do you have a crash plan? What happens if the site goes down due to load? Are you in a position where you can easily increase the size of your server or add more?

You may want to do some load testing, but it'll likely be difficult now before the launch. It takes a bit of coordination to setup for it...But it's something to think about in the future, especially for capacity planning.