Raiding is an interesting animal within the burning crusade. Unless you outgear content so significantly that it becomes trivial it has been said that every person in the raid needs to bring their A-game for a lot of encounters (sunwell probably being the case to point).
To a certain extend this makes sense. If you're in a bid to save the world you need your 40 or 25 or even 10 people to be the best they can be in order to defeat a high level boss and if they're not. You fail.
You don't want every fruitloop group that comes along to be able to kill illidan, magtherion or kael'thas because that would be game breaking to those who invest a lot of time, blood, sweat and tears into the game and would invalidate content at a pace too fast for development to keep up.
But the requirement to bring your A-game has some very disturbing consequences: They don't take into account personal emergencies, illness or untimely disconnects.
If your entire raid is dependant on everyone bringing their A-game then someone disconnecting will more often than not result in the entire raid wiping and a personal emergency can spell doom to a good night of raiding.
This puts more pressure on the rest of the raid who all of a sudden will have to bring an A+ game to the table to make up for the difference.
But other than those immediate consequences of wiping a raid there are also a lot of 'unexpected' side-effects. If you need to field an entire raid with players that can hold their own then all of a sudden you're looking at stringent requirements in terms of gear, time availability and even skill in order to have a good shot at progress.
This means that the friends of the guild, the somewhat less skilled, the undergeared and people with lower reaction times will either have to be trained and geared or sidelined for the raid.
This in turn puts pressure on the guild to gear and train people to a level where they can participate in raids.
Strap these training requirements to usually hefty raid loads (it's not uncommon for guilds to raid every day) and farming requirements for consumables and you end up with a very volatile situation.
A situation where long-term raiders are prone to burnout. Very few long-term raiders have the patience and the thick skin to go farming and gearing up lower geared players each and every day without them seeing any return in their investment. They could be training someone who is never going to be able to bring their A-game, they could be gearing someone only to see them wander off once they're in T5+, they could be bringing people to progression raids that simply are too unexperienced and as a result prevent progress.
What inevitably follows is burnout, leaving an even greater gap of gear and skill to fill in for your guild thus resulting in more pressure on everyone else.
A game mechanic that requires a set group of people to always bring their A-game is a mechanic that is prone to failure. If you cannot make up for random disconnects or personal emergencies without failing a raid then there's something inherently wrong with the mechanic. This in the long run will simply result in less people willing to deal with the mechanic.
There needs to be room for mistakes. Not necessarily a lot of room. But an instance should only require a B-Game so that if things go pear shaped the people that did bring their A-game will truly shine.
Until that is the case (again) you might well be excluded from end-game content merely on the basis of a bad internet connection...