At The Good (thegood.com) we usually start not by looking at data, but rather by talking to the actual users. Funny thing is, they will actually show you exactly where the money is being lost.
Look at where they are deserting on the site, where they get stuck in the conversion process, and where they are frustrated.
You'll also learn through testing that consumers/users have goals. And they are almost never the same goals as the marketing team who put the site together. So go find out what the consumer's goals are (by asking them directly) and then use that to make changes to the site.
After you've done this process, THEN start tracking and reviewing data. Otherwise, you're just looking at data for an unoptimized site... and mostly wasting your resources.
So in short, qualitative and quantitative. You need both, most forget one.