The biggest thing is the unholy horror that is negotiating with car dealerships. How do I negotiate with a website? If I am not negotiating, am I getting as good of a price as I would have if I was?
Additionally, I need a really simple/straightforward layout of what is or isn't included in the price and financing terms. Too often I had dealers focusing on the "monthly cost" while de-emphasizing the total cost, term of the financing, etc. That went not just for the car itself, but the add-ons like extended warranty and what-not upsells.
I want to see the total cost of the car (as negotiated), the total cost of any add-on features, and the total cost of the financing package selected (interest over the term), and then the monthly cost of each over that term below it.