1. Visits 2. Bounce Rate 3 Avg. time on site 4. Avg Page view per visit 5. Demographics
Anything else we should consider?
Check out Mike McDerment's talk on metrics: http://vimeo.com/10733370
Shortly after launch of our own app, we created a marketing table in our database. One metric we implemented immediately is to find out where users are coming from. Later on, as an example, you can join that with your user table and know where the paid users initially came from.
We also tracked how many times user logs in per day. From that, we started tracking many of the actions within our app. Now, using another table, we log almost all of the user's actions to get a better idea of how the user is flowing through the application. At the end of the day, track what you think will be useful to you and don't be afraid to track more than necessary.