First I'll define the term 'PUG Raid'. There is a difference between a 25-man raid formed of experienced raiders and good friends, and a 25-man raid formed with 25 complete strangers. When I say PUG Raid I'm referring to a raid with 25 random people, or pretty close to that. Most of us have been in one at some time or another, whether it's to raid Naxxramas, the Black Temple, or even a pre-TBC raid. These raids usually fail pretty miserably, and while there's not much that can usually be done to improve the outcome, there's several lessons to be learned.
Lack of Etiquette: Bringing unknown people in for their first raid often sets the entire group up for trouble. There's a lot of etiquette involved in 25-man raids. All it takes is one Leeroy wanna-be to face pull and ruin the experience for everyone. When you have 25 people raiding together, you're not wasting one person's time when it goes sour. You're wasting everyone's time. At the minimum, any person invited to a raid should at least be known to have manners. When you invite someone to a raid, it is good to make sure at least one or two people know the person isn't an ass.
Lack of Gear or Skill: Some people will never be able to dodge void zones, death and decay, rain of fire, or any other types of "circles of death" no matter how many epics they are wearing. There's just nothing you can do. For bosses such as Archavon, if you bring in too many newly-dinged 80s who can't play their class, you sometimes risk not being able to beat the 5 minute enrage timer. A minimum amount of DPS output and skill is required for raiding. While inspecting someone's gear won't tell you if they can avoid circles of death or do a reasonable amount of DPS, it's the next best thing to having someone vouch for them.
Lack of Strategy Knowledge: If someone's well versed in raiding etiquette, is a skilled player, and has good gear, this usually isn't much of an issue.
PUGs work great for 5 man instances. Not so much for 10 and 25 man, it seems. Any raider who knows what they are doing will most likely be inclined to join a guild where anyone who does not possess raiding etiquette or a fair amount of skill is not allowed inside in the first place.