I think the main difference is in the comb - Dominiques have a pea/rose comb and Barred Rocks have a straight comb. There's also a diiference in the feathers. Dominiques I belive have a more even barring... but I'm not totally sure.
I think it's just the difference in comb. Dom's have a rose comb and Rocks have a single comb. If you click on the BYC "Breeds" section you can read up on it all you want.
If you are talking about Hatcheries Stock then the only real difference is the comb, Dominique have Rose Combs and Barred Plymouth Rock have single combs.
Now if you are talking about Show Stock then there are a little more than just the Comb being different between the two breeds. http://www.dominiquechicken.com/Dominiques_and_Barred_Rocks.html