The one thing I've commonly heard is that you can usually see more feathering around the ears. This is one of the first good indications as a puppy. Like Michael mentions, there are different degrees of fluffy coats. Plus AKC will only offer limited registration to fluffy corgis (meaning they don't encourage breeding).
If you talk to most fluffy owners, they will tell you fluffies are a very special!
Many will tell you they have a much nicer temperment, but I don't know if there's any truth to that.
My Gizmo is a fluffy, he's very striking, and folks always want to stop and pet him. I always get asked what breed he is, which several think he's a sheltie.
Even though the coat is a flaw of the breed, you will find several fluffy lovers. I'm on a yahoo list called "fluffycult", some them have more than one fluff, and if they lose their beloved fluff, they usually try to get another.
I took Gizmo to Corgi Nationals and met several folks that had top show dogs, but they told me they had a very special fluffy corgi at home.
I know they claim they can now genetically test for the fluffy gene, but I think most folks love for them to occasionally show up in a litter. This isn't the type of genetic flaw that can cause any physical health problems to the dog. You just might find yourself grooming your fluff more often.
Here are a couple links. This first one shows how my fluff looked as he grew up:
http://community.webshots.com/album/245730048gQodQB
Here is a link to Pembroke Welsh Corgi Club of America and coat descriptions:
http://www.pembrokecorgi.org/CorgiColor.html
If you want information on joining the yahoo group called "Fluffycult" just let me know and I'll get you the information.
Fluffy Power!