Saying a game should be fun is like saying food should taste good; it’s trivially true but effectively meaningless when you want to discuss how to make food.
Just like you have to prepare different foods in different ways depending on the experience you want the dinners to have, you have to design rpg mechanics in a certain way to flavour the experience of the players. Worse yet, unlike food, you can’t advertise this. RPG consumers are some of the most fickle and truculent people in the world. They don’t want to know how their sausage is made. They want to pick up and play the game and have it just click, unconsciously.
This is why, for example, White Wolf designs its character creation system the way it does. It’s not designed to produce a good balanced character, it’s designed to produce an organic character, one that results from deliberate choice. The game system would almost certainly be better served with a pure point buy mechanic or a series of rigid archetypes if your goal was balance, but it’s not. The goal of the White Wolf character generation system is to force players to think, at each step, about what their character would be like. “Is my character brainy, brawny or beautiful?” “Do I focus on action skills or knowledge?” and so on and so forth. The system is designed such that you can never monofocus in the area you want to focus the character as well, so that you are forced to distribute points in other areas you may never have considered. “Damnit, I already maxed out Melee and Dodge and Resistance and Athletics but I have like, twelve points left… uh, Lore I guess? And some Occult, why not. Medicine as well…” Even the way it breaks down the difference between BP and XP is important, since BP allows you to hyper-specialize while XP encourages you to pick up small increases in a number of traits over time, leading to more organic-feeling characters.
The feel of the game, one where you create a deliberate character rather than a collection of combat stats, is much different from D&D (where “Race, Class, Done” is a valid and entertaining character creation system). And most of this doesn’t occur to the average player, anymore than most people stop to think about what ingredients went into the cake they are eating.