A simple enough question. If we take any given style of whisky on the lower end of the spectrum, and we compare it with a fine Scotch, what is it about the production of the two bottles that causes one to be more palatable and easy to drink than the other?
This question may end up being closed as too broad, as a full answer to this would include many lifetimes of experience by master distillers, and would have to incorporate opinions (some people find peaty, smoky whiskies absolutely foul and undrinkable while others would not ever want to drink a mild lowland whisky)
That said, I'll summarise my most critical ones, and the thoughts that guided me when distilling my own whisky:
Aging: the cask imparts the vast majority of the flavour, from the type of wood, the previous contents, how charred the cask is, length of time in the cask etc. The more desirable whiskies carefully choose casks and aging to deliver tastes that customers will like. This typically costs money (especially for storage costs) so a cheap whisky may not put so much effort into cask selection, quality control, age etc.
The Middle Cut: The foreshots start off with mostly methanol so would not taste good, and are mostly poisonous, but a small amount of methanol is useful in the process (and evaporates first after casting as part of the Angel's Share anyway) so the decision when to extract the middle cut is essential. Similarly, the feints, which contain propanol and heavier fusel oils are not wanted so are fed back into the spirit still. But there will be a small amount in the final product. Knowing how much of the foreshots and feints to include is part of the master distiller's craft.
Speed of heating / shape of still: Tall stills give a very smooth clear taste, especially when slowly heated. Short, squat stills combined with rapid heating allow many more fusel oils and other substances into the distillate, which will make the flavour more interesting, but can lead to a lack of smoothness.
Some cheaper spirits get around some of these issues by multiple distillations to remove some of the more complex substances - this can lead to an easier flavour for drinking, but also to something far more boring (from the perspective of a whisky afficionado) and bland.