Most important part of Google analytics

If you apply the 80-20 rule to Google Analytics and Webmaster Tools, what are the 3 most useful items in each?
What I mean by most useful, is a combination of greatest influence on hits and efficiency of improving it. For example: reducing bounce rate would be good, but it is rather hard to change if your site has already good content and a couple of enticing links above the fold on each page.
I’ve already gone through errors; html is virtually fault free and Google webmaster does not indicate any errors any more.

I only managed to get tools and analytics working this weekend, so know virtually nothing about it in practical terms. What should I concentrate on first?

I like to look at pages per visit, number of unique keywords driving traffic (that is a good indication that as your site grows are you starting to rank for more keywords), and average site speed (not the most important factor, but something to keep an eye on).

There are many more things you can track, but those are some quick high level things I like to check to make sure the site is performing well.