I've eaten at Le Cellier twice and enjoyed it both times, but voted for Marrakesh as a more entertaining and interesting experience - not to mention the fabulous food! The entrees are not highly spiced so the less adventurous diner doesn't need to be afraid, but the food is just different enough to make it more special than a steakhouse. Beyond the food, the decor is gorgeous and the musicians and belly dancer provide excellent ambience. I can have steak anywhere; when on vacation, and especially in a place like the "World Showcase," I'd much rather try something exotic.
ITA. Le Cellier is a steakhouse. It looks like a steakhouse - reminds me of the inside of Steak and Ale. The food tastes like a steakhouse. I am sure you can find an upscale steakhouse wherever you live with a similar ambiance, and just as good (if not better) steak. We like it, and usually eat there on one trip a year - but it is not the type of restaurant you can only experience at the WS.
Alfredos is good, but Mexican and Italian are probably two types of food/atmosphere that you can get at home.
That leaves the following:
Biergarten - loud and fun. You sit at long tables with other families. Very unusual atmosphere (A++) and it's probably difficult to find authentic German food at home. If you like beer, this is your place.
Chefs de France - The atmosphere is nice, although not as unusual or magical as some of the more heavily themed restaurants. It's not easy to find good French food everywhere. French food is not my favorite - but if you love it - Chefs is a good choice.
Akkershus - The atmosphere here is not that exciting - sort of like a scaled back inside of Cinderella's Royal table - but there are PRINCESSES!!! (Not sure your mom and dad are princess fans). Lots and lots of little kids and families. The food is extremely authentic Norwegian food - people love it or hate it. Lots of cabbage and cold smoked seafood. I imagine Norwegian food is hard to come by - so if you love it - Akkershus is worth the visit.
Nine Dragons - I have not eaten there.
Marakeesh - This is my favorite. A++ atmosphere/theming. I also love Moroccan/Persian food and it's hard to find. Depending on your parent's personalities, the bellydancers will possibly delight or offend them.
My preference for Marakeesh is really based on the fact that I just like the food. So, I'd say for an unusal dining exerience, I'd suggest Markeesh, Biergarten, or Chefs De France - depending on what type of cuisine your party prefers.