I agree that there is absolutely no comparison between a properly ripe Granny Smith and a supermarket one.
Some people's palates are far more sensitive to acidity and bitterness than others' and some people cannot detect notes such as nuttiness, pineapple or banana that give individual varieties their character. Such matters of taste are partly genetic (eg I was born a supertaster and make my food choices accordingly) and partly cultural - ie whether they live in a society that likes to make eating properties a matter of discussion.
For those who can't pick up lots of interesting flavours, the texture becomes all the more important, I imagine - crisp, crumbly, crunchy, juicy, silky, melt-in-the-mouth, and so on. There are lots of ways to enjoy a fruit! ;D