If you were driving Challenger SRT8's and/or Mustang Cobra Shelby 500's around all day, and weren't impressed, you must have been driving them like my mom. I drove that Shelby this past summer: crazy, crazy fast, and handling to match.
I'm guessing you were driving the SXT's/V6's.
It's not just performance. I just don't like the style of the car. American muscle doesn't appeal to me at all. You can put whatever you want in a mustang but its still a mustang and I don't like them. I'm much more into the import tuner cars. Evo's, STI's, Supra's, etc...