Wow, this is a great forum. I just wanted to post my 2 cents.
Charleston's is now closed. There is a new restaurant there that has great food also.
A couple of our favorites are:
Charlie's Crab - Great dinners, upscale.
Dosido's - On the south end, I grew up in the mid-west and moved to Hilton Head Island 4.5 years ago, the ribs here are the best ribs you will have in your entire life. You can literally shake the meat off the bone. Casual.
Boathouse II - right next to Charlie's Crab, this is simply great food. Upscale - Casual
The SeaShack - You most likely won't find this place unless you ask a local, we try to keep this one a secret from all the tourists. Very Casual.
For Pizza I recommend Guiseppi's or Mellow Mushroom, both great.
For a good sub, grinder, or salad stop by Stu's on the south end in the Coligny Plaza.
I hope you all enjoy your vacation!