Ah... Eggnog. I have tried numerous recipes, the best of which are all pretty similiar..... eggs , sugar, cream, booze. The major variant seems to be the % of booze which runs from 10% to as high as 55% in some recipes I have tried.
IMO what makes a major difference in flavor is aging. I typically age mine for 6 - 8 weeks.... drinking it through the holiday season from Thanksgiving through New Years. When first made the flavors are easy to identify... eggs, cream, sugar, booze. As the brew ages the flavors meld, the consistency thickens and the mouth feel becomes very silky.
I use at least 20-25% alcohol and store the eggnog in the fridge below 40F. I would not recommend aging an alcohol free eggnog.