Nothing!
I spend huge amounts of time in my car (50k+ miles per year) and have loved every minute of it. This car is a wonderful blend of performance and economy--I drive rather quickly but still am getting ~29MPG. The only downside I have seen is that short people have a hard time with the visibility (I don't, but I'm 6' 5").
I've driven both the 4 and 6 cylinder Fusions and found the 4 to be a very well built, solid and great performing car. It is faster than the pre-2007 Camry V-6, and much faster than the new Camry 4. Ford definitely has a winner in the Fusion, and they are selling here as fast as dealers can get them in, now that gas is so expensive.
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