In a recent comment a reader asked me to define "casual". That is actually a tricky question, because I've never seen a clear definition, and most people disagree where to draw the line between casual and hardcore. One solution is add some sort of middle-class, the semi-casual, but then again you can start argueing where exactly the borders are between casual, semi-casual, and hardcore.
Many people try to base their definition on hours played. If you play only 1 hour per week you are definitely casual. If you play 100 hours you are definitely hardcore. But at what number of hours per week do you define the border? 10? 20? 40? We'll never agree, because counting just the hours doesn't completely cover the problem. It is only a starting point. You have to ask yourself not only how many hours do I play, but also what would I have done with those hours if I hadn't played?
So I would define "casual player" as somebody who uses the game as entertainment for his spare hours, but for whom the rest of his life always takes precedence. While "hardcore" is somebody for whom the game has become a greater purpose, and he is willing to make compromises and sacrifice time he would otherwise have spent studying, working, or for real life social activities. The difference is not time spent, or skill, but commitment.
That definition explains a lot about the respective attitudes towards raiding. The problem with raiding for the casual player is often having to be online at a specific time and for a specific block of time, together with the other raiders. If each raider is only playing when he has the free time for it, it is impossible to get a raiding schedule organized. And if you organize 10, 25, or 40 players for blocks of several hours on several nights a week, it is impossible for everybody to attend without making compromises with his real life schedule.
To be "casual friendly" a game needs to have a lot of solo content, and it must be possible for a quickly thrown together pickup group to advance through the group content. That limits the difficulty the game can throw at the players: If group content needs a specific group mix it takes too long to organize groups, and somebody having to leave a group for some real life incident turns into a risk for the whole group. If the group content becomes so hard that people need to practice the same encounter repeatedly and learn how to coordinate with a specific group of people, playing with pickup groups becomes impossible. The impression that casual players are not playing as well as hardcore players is only true on a group level, due to the necessity for the casual player to play with whoever is available. On the individual level the casual player is playing as well as the hardcore player.