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For me I think it could be shipping costs or product comparison. When I am on a shopping mission I can have as many as 5 or 6 sites with the same products in their shopping carts comparing prices and shipping costs.
On my personal site I see about 1% of the carts are abandoned.
Another cause of abandoned carts could be a convoluted checkout process. If you make the checkout process as simple and effortless as possible your abandoned carts will decrease. One site I worked with had a couple of fields in their credit card information portion of the checkout form that I had never seen on any other shopping cart. I can't remeber exactly what they were but I had them remove those two fields and their abandoned cart rate decreased dramatically.
As far as other metrics to look at, one that sometimes tends to get overlooked is the lifetime value of a customer. Many times people equate a customer to one purchase but many times if you get that customer and they are happy they will be back to buy more stuff. Rather than looking at a customer being worth their $20 purchase try to figure out how many $20 purchases that customer will make each month/year/lifetime and assess a customers value that way.