On every car I've ever owned, 1983 and newer:
The stupid, useless, plastic freaking undertray. They are always broken (yes, even on my all-of-5-years-old Mazda5), they are always crap collectors (leaves, grime, rocks, mice), and they are always a hinderance to oil changes (blocking filter, blocking jack points, etc). I have a literal pile of them in my shed (Miata, 944, Mazda5, Grand Prix). I flipping HATE those things.
On the Mazda5:
The windshield wipers: The fronts only have the choices of "once every 3.14 minutes", "rainstorm deluge", and "OH MY MONSTER, WE'RE ALL GONNA DIE!!!!!!" for their settings. I like wipers that have normal settings, or even better, a potentiometer switch so I can adjust on the fly.
The rear wiper: It sucks. It pretty much just streaks the bottom 1/5th of the window, on the passenger side only. I've replaced the arm itself and the blade and adjusted the sucker every which way it can go. After going to a few dealers, I've realized it's designed this way. I'm gonna take that berkeleying thing off.
The Grand Prix:
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The automatic. If GM made that car with a decent 5-speed stick, I'd own a fleet of them. Not that the auto is that bad, but in a FWD sedan it needs a clutch pedal to have a little fun. I've driven a buddy's Tap-Shift equipped Series V GTP and it was better, but still lacking.
The 944:
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The shifter. It has the feel of a broken leg with the directness of a bowl of mashed potatoes. Maybe I've been spoiled by aftermarket-equipped T5's and Miata's, but that shift action feels like rowing a canoe in a river of broken glass.
The Javelin:
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The vinyl top. I have 127 trim holes to weld up. Stupid 70's fad.
Sorry, I'm in a bad mood today. I'll go sit in the corner and be quiet now.