One of the problems with PUG raids is that raid content is generally accepted to be too easy but in reality much of it is too hard for new and casual players. Some time ago, Ardol at World of Warcraft Philosophized posted about why the leveling process doesn't prepare you for raiding. He makes some good points about the differences between solo and group play that contribute to why people manage to be profoundly bad at raiding with their class despite having to go through the leveling process just like everyone else.
Much more importantly to the issue of PUG raiding is that the leveling process, in its current form, doesn't give you the most important skill in raiding: dying, running back, asking yourself what you can do differently next time, and trying again.
In raiding, you have to be ready and willing to lose and to learn from your mistakes. Without this, you really don't have any opportunity to improve your play. But when raids break up after a single wipe, you never have an opportunity to get better even if you are ready and willing to analyze your play and practice.
When a fight isn't explained and a wipe results in everyone just giving up instead of discussing what went wrong, a newcomer to the fight won't even know why they wiped or whether anything could have been done to avoid it. Sometimes to an experienced raider the causes of wipes can be pretty obvious even on a fight they've never seen before. For a new raider, however, how do they know the difference between - to use Toravon as an example - the raid losing because too many people stood too close to a pair of frozen orbs and the healers weren't on the ball; versus the raid wiping because the fight went too long and he got off a lethal white out? Their experience of either will be similar: everything was going fine until suddenly it wasn't and nearly everyone died.
The problem this creates is that not only does leveling not teach you how to raid, raiding doesn't always teach you how to raid. Unless you are raiding with a group of people who actually share their experiences and who are willing to try again, you are just spinning the wheel every time and hoping you come out a winner.
Dying in WoW is of little consequence. Usually it won't waste more than 10 minutes of your time. Using 10 minutes to die is a great use of time if you actually manage to learn something from it.
People often say that WoW has become too easy, but I want to be totally clear about something: WoW is shockingly hard, but the number of things to do in WoW that are actually really that hard are fairly limited. Firefighter, Yogg +0 and heroic Lich King are all very hard. Algalon, heroic Anub'arak, and heroic Sindragosa were pretty challenging as well - even if the former two seemed harder than they really were thanks to limited attempt mechanisms.
But there is a distinct lack of build-up to the encounters of that difficulty. There are parts of the game that don't seem that tough and then the difficulty curve suddenly gets very steep. PUG raids usually want to take on content up to the point where the difficulty ratchets up, but don't want to try to go further because they can't reasonably expect a PUG will win. Because they only take on the content that is "easy" they expect they should win without a great deal of difficulty. This adds to the frustration of losing, increases the chances the group breaks up after a small number of wipes, and reduces the learning opportunities for PUG raiders.
Reversing this trend means having to start early. Players should get used to the idea of dying and trying again before level 10, and should be reminded that that is part of the game as they go through the leveling process. In his inaugural developer blog post, Ghostcrawler points out that the fun of playing on God Mode wears off pretty quickly. That philosophy needs to be applied consistently through the game, not just to heroic raid encounters. The player base should expect that they will need to try more than once to accomplish important things, and they should get used to feeling that good feeling you get when you succeed after failing, learning and practicing.