1. Revenue/Orders (conversions)
1B. Costs (cant track ROI without knowing your costs)
2. Actions: Subscribes, Surveys, downloads... (lead generation)
3. Unique Visitors - real humans, not bots or junk traffic (your pool of opportunity)
4. Link baiting... shares, likes, backlinking, social bookmarking (growing the pool of opportunity)
Obviously #1 matters most... which is why they call it a "conversion funnel"
1-3 are easy to report on and track progress towards goal. You have everything you need in Google Analytics and other similar solutions.
4 is difficult to report on but you don't want to neglect it even though most businesses fly blind.
Note... I say orders and revenue for number one because you can increase profits by increasing number of orders OR the average order size. You need to track both to seize those opportunities.
If you are fulfilling your own orders then turns is important too. This will help you keep investing in things that sell and liquidate things that don't so you can put the funds back into things that sell.
If you sell apparel it is good to track turns down to the specific sizes. You want to stock the sizes proportionally to how they sell to your audience or you will sell out of XLs overnight and sit on a pile of XSs that few can wear. Same holds true for colors... you will find different proportions of desired colors. Blue and black will sell out fast and Orange will linger for years.
It should all turn into a feedback loop that continually helps you learn and sell more.
Optional Story:
I once set up an online jewelry store where I would source glass necklaces from China for $0.25 each. I would sell them for $15 with free shipping in the US. I calculated my fully landed cost:
$0.45 Unit Cost + Shipping and receiving costs
$2.15 USPS Shipping with Tracking Code
$1.15 Shipping Materials
$1.25 My time to pick and pack
$5.00 fully landed cost.
I knew going into it that I could completely replace an order with no questions asked and still make 100% profit.
Obviously marketing costs get applied as well to determine final ROI by marketing channel.