The goal of any game is to provide entertainment. Games are a diverse medium and can provide entertainment in a number of ways. I think you can break down the various ways to be entertained in a game into two main categories: gameplay and non-gameplay.
Non-gameplay aspects of a game are aspects that the player can not interact with in order to "win" at the game. This could include the quality of graphics, pretty scenery, the music, the art direction, the atmosphere, chatting with other people, etc. It also includes being able to do "cool" things in the game - anything that evokes a sense of wonder in the player.
Gameplay aspects are those factors that depend on player input to decide whether they will "win" at the game. From tricky fast-paced arcade timing of moves and abilities, to strategising on the best way to solve a puzzle, this stuff is what we mean when we talk about gameplay.
In good games, the same feature of the game features both aspects. For example, in Super Mario Bros 3 I can obtain the Raccoon Suit. The Raccoon Suit is powerful. It lets me fly for short bursts, slows my descent when I fall, and lets me hit enemies that I couldn't kill by jumping on. The Raccoon Suit is also awesome. It gives me little ears, my tail waggles, it makes cute sound effects. I'm also not used to flying around in a game and defying gravity and ninja-hitting people with my tail, so I am left with a distinct cool impression resulting from the unusual gameplay.
This is a fantastic marriage of the various ways in which I can enjoy a game.
Then we slide down the spectrum of well-designed games to, oh let's say, WoW. WoW features a lot of good non-gameplay aspects. Cool looking characters and powers, giant world in which to interact with people and cool looking places to explore. But what about the gameplay aspects? Tobold gives a nice run-down of these
Basically encounters boil down to: having good gear, prior knowledge/experience of the encounter, the right preparation (knowing build and skill rotations), 'arcade-style' reflexes, and tactical use of skills.
So what's the problem? Lots of gameplay there right? Well... let's face it. Players are lazy. They look up which gear to get, and then they grind until they have it. They youtube the encounter, or read some guides, and then use rote memory to get through. The build is also obtained from some website, along with the best skill rotations. Even players that don't look this stuff up on a website will have it forced on them by other players in their guild that do, as the other players expect them to contribute optimally.
Ok, so we're left with the arcade-style gameplay and the tactical use of skills as possible avenues by which the player can actually affect the outcome of the fight. The problem: WoW uses a GCD system, and so everything happens really, really slowly. Anybody can push the buttons in time. I don't care how old Tobold is or how advanced his arthritis is, he will always push the buttons in time - if he knows which buttons to push.
So the problem is reduced to, knowing which buttons to push at the appropriate time. I'm not talking about 'optimal skill rotations' here - I'm talking about what needs to be done at particular times to respond to a dynamic situation.
For a new player or for someone who doesn't read the forums, performance on this aspect can be pretty sub-par. This is due to the massive amount of information required for the player to know - they have to understand all of the game mechanics and the functions of many other skills and classes in order to make the correct decision.
But once this information is learnt, a game like WoW doesn't require much further tactical knowledge. Encounters don't change. Aggro operates by easily predictable rules. Sometimes you may have to decide who to heal if people mess up the aggro, or perhaps move out of some glowy stuff on the ground. There's no pressure vs spike considerations and there's little in the way of protection skills, meaning that you rarely even have to watch the gameplay to decide what to do next.
In short, it's pretty hard to mess it up once you've pre-learnt the little bits of information necessary for success.
Don't get me wrong, it's still an entertaining game, what with the shiny pixels and everything. It's just not really an interactive game. Tobold is quite correct in that the design approach makes it as accessible as possible to as many people as possible. But without really being challenged to interact with the game in *some* way to be able to beat it, I just can't seem to make myself enjoy it.