Ever loved a conceptual design from an automaker only to be let down by the actual model that rolls off the assembly line? Things got a lot tougher for automakers in the last 30 years: safety, insurance, emissions, global platform sharing, etc. all make car designs far more constrained than they used to be.
Here's my biggest letdown, the Pontac Aztek:
For me it was the current generation Lincoln Continental. I think it was Bob Lutz who said it looked like they took the concept and left it in the oven too long and melted the sharp fender edges down.