When I ran my first D&D 3.0 campaign, the new rules had just come out, and we were all exploring them and having fun with it. The dorms were full of Mountain Dew and Cheetos, and we had seven guys, three books, and an epic quantity of dice.
They all made characters, and I gathered them together, and looked them over, and I immediately saw a huge discrepancy in the way that they had been designed. Little did I know that the lesson that I learned that day would become one of the more defining elements of my life as a gamer and an intelligent human being.
Without going into the rules of D&D, I'll use two characters as examples. Both were intended to play the role of "Former High Guard to the Deposed King".
Character 1 was a 7th level Fighter.
Character 2 was a 2nd level Fighter/1st level Monk/2nd level Ranger/2nd level Paladin.
The reasoning behind each choice was clear, coherent -- and utterly different. Player 1 said "I want my character to fight -- and what could reasonably fight better than high-level Fighter?" Player 2 said "Hey, the rules allow me to multiclass, and every fighting class is frontloaded, so if I take the earliest levels of each, I'll end up a much better fighter than any single-classed character."
Player 2 was right; his character outperformed Player 1's character in every combat. So much so that Player 1 objected -- and he did so by claiming simply that "There's no backstory that could justify that motley collection of classes." Player 2, who had never even considered a backstory for his character, responded by asking where in the rules a backstory was required. The argument between those two perspectives has never ended to this day - and I see examples of it everywhere.
Thus, I discovered first-hand the chasm that exists between Mechanics -- players who are interested primarily in what the game's rules require and allow -- and Aestheticians -- players who want the appearance of the game to matter just as much as the rules.
For the record, after seven years of DMing, I'm a Mechanic. And here's why. The appearance of things can totally make something that's unpalatable much more acceptable -- but without a functional core, no game is playable, no matter how pretty. I can't tell you how many times my wife and I have purchased a beautiful and amazing new board game only to run into a circumstance that the rules don't cover in the first three times playing through. It sucks. I'm a firm believer that the rules are the most important -- and frequently the only important -- thing about a game.
So what?
In many games, there are a lot of people who can't seem to distinguish between a mechanic and an aesthetic. Let me explain: a mechanic is something that actually allows or prevents a particular action in-game. An aesthetic is...everything else.
The aesthetic point of view is totally valid, totally true...and totally irrelevant to anyone actually playing the game for the game's sake.
It's precisely for that reason that I've now given up D&D entirely (well, 4th ed. helped)...and taken up Everway, an RPG that is mechanics-lite so they don't get in the way, and actually takes aesthetics into account as part of the core mechanic of the game (the book virtually says "when the DM is making decisions, he should always take into account what would be awesome." -- can't go wrong with that.)