I have an 01 4x4 ST and love everything that is on it, but would love to have an 08. So I'll base mine on the new ST.
Best: In dash GPS system. Also the rear DVD entertainment system. I have them strapped on the headrests in my 01 and this is a must, since I our 8 year old grandson lives with us.
Absolutely the Worst: This is not an option, but I think this is the worst, is having the gas fill on the passenger side and not on the drivers side. Even so I'd still buy a new ST if could.
One option I would love to see, is the movable type running boards similar to whats on the Navigator. You don't see the running boards till you open the door, then they pop down and out. I have seen where you can order them as after market from different parts stores. The running boards are great getting in and out of the ST, but at times they do get in the way. It would be great to have hidden running boards that pop out and down when you open the door.