These people are full of fail. I see three ways to categorize them: fail, unsure, and social. People can fall into more than one category.
What's so bad about them? Their behavior and attitude are entirely the opposite of what is needed for the non-hardcore guild. They expect everything to have been dead last week and cannot handle the idea that some people learn more slowly than others. They lash out. They ragequit. They blame everyone except themselves, because they think they are awesome and aren't quite sure why they're hanging out with these losers.
I believe that most community problems can be attributed to these sort of people. They're the ones making poorly-run PUGs demanding 5k GS for regular ToC 10. They're running trash DPS meters in randoms. They're screaming about how everyone sucks, whether it's in a BG, raid, or just trade chat. They're often the ninjas, since you're so awful compared to them that they deserve it all, and maybe it's your fault that they are in this stupid random instead of fighting HM Arthas.
They are that grouchy guy at work who hates his job but isn't quitting it any time soon. That guy needs to shut up, doesn't he?
The three categories have different patterns of negative behavior, so not all of what I said earlier in universal to them.
In the simplest terms: this person isn't skilled enough to be in a hardcore guild. Maybe he was in one and got kicked or driven out. Maybe he applied a bunch of times to a bunch of guilds and got into none. Maybe he was in a casual guild which changed and he lost his spot. Whatever it was, the world has been bluntly telling him that he's bad. He didn't listen and is certain that he is amazing and the rest of the world just sucks, especially his current, non-hardcore guild.
Expect excessive aggression, blaming, and little patience. This player may also try to convert the guild.
This player really wants to go hardcore and kill Arthas HM and dreams of someday doing a 40-man raid on GM Island. Rawr says his DPS should be at around 9.7k in his gear, but he's struggling for 9.4k and thinks that's terrible. Every wipe crushes his spirit as he runs through the ten thousand mistakes he made. This doesn't mean he truly made ten thousand mistakes, but he is certain that he should have been half an inch in that direction at that time and that would have saved the raid.
Expect a lot of emo, self-deprecating apologies. Beware, may start to ask why the rest of the raid isn't fixing their ten thousand mistakes and become angry.
This player likes the people he raids with and doesn't want to lose them. Fails to realize that he only cares about a small portion of the guild anyway and will still be able to whisper them or talk in custom channels. Hasn't fully grasped the notion that he had friends before the guild, made friends in the guild, and probably could make new friends after the guild.
Expect a lot of chatter and attempts to keep the raid motivated. Warning: May suddenly realize he secretly hates half the raid. Don't worry, he's too afraid to hurt the other half to say anything.
These can all overlap, with increasingly obnoxious results. For example, the social fail combination may form a clique which blames the rest of the raid. The fail unsure combination might leave to go cry in the middle of the raid because not only does he make ten thousand trivial mistakes, he also makes ten thousand significant mistakes, and he knows it. The social unsure has no mic and worries that he might have been wearing a pirate hat this whole time.