Well, I'll give you a better idea of what this is like, and why people care. To compare it to non-MMORPG games, Arthas is like the final boss. His raid instance is the final dungeon. All of the major plot points reach their climax there. Specifically, the major plot elements which started in Warcraft III, and have been touched on by a lot of quests and things along the way in WoW.

Now, it used to be that, in order to fight the enter the final dungeon and fight the final boss, you had to organize a large group of 25 people, create a schedule, dedicate a lot of time to play (coordinating this time with the schedule), and slowly work your way through other dungeons, and into the final dungeon.

So, casual people are often upset that they can't access to content which houses most of the well-known story characters and plot developments, and so they tend to resent raiders. Raiders, on the other hand, are often happy with the fact that they're the only ones able to see this sort of thing, and resent casuals for trying to get stuff "ezmode".


The fact is, most casual players don't want the game to be easy, *just more accessible*. They want to be able to play the game without having to coordinate schedules with 24 (or, God forbid, 39) other people, or dedicate a huge amount of time in single sittings to complete something.

Including 10-man versions of raids make them much more accessible to casual players. Raiders don't like it, because they complain just about any time the casual players get to do something that they used to have all to themselves.