The difference is not astounding but is nether the less there. At the HNY you are given real metal cuttelry and real napkins, also real tablecloths (ok, not hugely important but a nice touch just the same!), unlike the SL, NPB and the good morning fantasyland breakfast. Here you get plastic knives and forks, paper napkins and paper or no tablecloths. On the food front at the SL, NPB or fantasyland breakfast you only have one choice of juice - orange. At HNY you can have orange, apple or grapefruit juice served from nice silver holders. There appears to be a larger choice of cereal. There is more fruit available also. However both breakfasts share simalarities of bread, croissants, cheese, ham, fruit salad, yogurts etc.
I dont know why you cant do the fantasyland breakfast if you stay there, maybe its a perk for the other hotels. Its probably that people who stayed there would complain on the quality of the fantasyland breakfast if offered. Also the hotel appeals to older people who may not want the fantasyland breakfast? Anyway when i stayed there it was offered me as a "special offer" when i arrived. I took the offer and wish it was permanantly offered at HNY.