Checking the link posted aboveis probably the most complete list but here are some things off the top of my head.
-rearwheel disc brakes
-engine oil cooler
-HD hoses (radiator heater etc.)
-Re-enforced front lower control arms
5/8" ball joints (civies have 9/16)
-Heavier front brake pads
-Body bushings already in place
-PCM programmed for a higher speed limiter(130 as opposed to 108)
-Gear ratio may already be 3.42 (civie is probably 2.56, 2.73, 2.93)
The civie has a nicer interior leather, pwr everything. Most 9C1s are spartan as hell. But if you find a nice 9C1, it is easier to spruce up the interior than to bring a Civie up to 9C1 mech specs. My $.02 worth.
Oh and one othe thing, a nice Caprice will cost between 8 & 10 grand (depending on location) and a 9C1 can be anywhere from 1500 to 5000 bucks.