There are many MMO's out that feature an endgame activity known as Raiding. The idea is generally that Raiding and it Dungeon associates will serve as the endgame for PvE encounters. Raiding is suppose to be where your story leads to or finishes in a given expansion. Since Raiding is tied directly to the story progression of a game it inherently features the most influential and powerful characters a game will have to offer. This means for the players to accept the story of the game these characters must present believablilty in their strength to be in the situation they are in. If it only took a hard poke to kill Arthas players would stop caring about killing him. This is why raiding kills drop down significantly on older content. If it is way to trivialized it breaks the game. Generally this requires expansion level trivialization to break older content. So in general people need to believe what they are participating in should defeat the enemy they are raiding(unless its Prince Kael). To this end raiding features larger numbers of players to accomodate immersion then most experiences players take place in.
So now we come to the part of designing the raid. This is where the Holy Trinity of gaming comes in. Damage Reduction, Damage Healing and Damage Dealing. Incredible variations of these three elements is what makes up both the Boss fights and the players who participate in the raid. These three elements are presented to the players at a certain level of challenge to make the game fun. The most note worthy thing about all Raiding I have encountered is that the skill requirement of each element of the Raid is fixed for each encounter. The players however are tossed at the encounter rather randomly in that for example the same tank may not mitigate the same amount of damage as he would in the same encounter due to mistakes or even RNG. Since mistakes impact the player level contribution of the three elements of raiding there is then the input of player skill in determining raiding success. It is also note worthy that to fail in any element of the raid will cause the party to be defeated. In a previous post I broke skill down into its components. These were a goal, knowledge, execution and adjustments to knowledge. Raiding is therefore the goal to which you must have the knowledge of the encounter mechanically, be able to react to using the knowledge of the encounter and be able to make adjustments based on the dynamic qualities of a raiding party. This is usually how raiding plays out in a very simplified way.
Since trivialization of content is often a measure of skill relative to the fixed elements of an encounter we can assume that for the encounter to be immersive and therefor fun the raid must offer a skill level suited to the players in the raid. Since these skill levels are not constant in non-exclusive groups encounters that don't offer variable skill levels within themselves will cause continued failure when skill is capped. It is also easy to assume that only exclusive groups will be able to handle high skilled content. Randomized grouping prevents high skilled content from being possible.
To make a raid what you are trying to do is hit players below their skill if they were first to attempt it and then by the process of them reaching their skill cap for the encounter overcome it. Simply if they overcome it immediately they won't have fun, nor would you be able to keep up content production in all likelihood. If they can't overcome the encounter they will eventually stop having fun as failing completely sucks. By hitting that sweet spot of difficulty people become better at the encounter and complete it over a longer period of time while having a blast. The core of this is that if you expect to be bringing in a random of selection of skilled players you need a more varied option of skill components of the encounter to facilitate the players brought. That is how you make a great raid.