1. Make sure everything works. Test fringe cases. Intentionally try to break your site. Hit the order button three times real quick and see if you triple charge an order. If you did, did you create three orders in your database or just try to charge the card three times.
2. Understand how site actions and form submissions get recorded in your database. Make sure you are seeing the data that you expect to see. Look at things on a line item level.
3. Remove ambiguity. Make sure there is a clear relationship in the data that you are capturing, and that all tables that need to have joins between them do. There is nothing worse than having to make a leap of faith because you tied something to a user that should have been tied to their order.
4. Understand what data can be overwritten and what has to be kept as historical. For instance you may have an address table that can be updated, but if you do that make sure that you have the billing and ship to address from an old order stored on the order table. If not you may not know where you shipped something.
5. Be able to distinguish between robots and real people in your traffic.
6. Set up a table that records all of the user cookies you issue. This table should record things like the IP address of that cookie, their referring_url to the site on their first visit, user agent, and initial url on their first visit.
7. Be able to tie important site events back to user cookies. This way if someone places a fraudulent order on your site you have the IP address of the person placing it. Have a bug that only seems to affect some people? Check their user agents, maybe its a Netscape or older browser issue.
8. Balance the online and offline. If you took 30 orders, did you ship 30 orders, and did you get paid on 30 orders?
9. Establish the lifetime value of users so you know what acquisition costs you can afford.
10. Segment the hell out of your users. Look at things by source, by new and returning users, by entry point, by product line, etc...Things rarely convert uniformly across the site. Segmenting helps you know what is really working and who your best customers are.
11. Figure things out yourself. Its your business and only you are really going to know what you need to be keeping an eye on. I strongly recommend knowing the ins and outs of everything your site does and records and working your way up from there. You will find that somethings you paid attention to early on are not important, but thats part of the process. When a crisis or something funny comes up, the better you know your site, the easier it will be to troubleshoot.