Well, the negative human element that is ruining the hunts for others (myself included).
I agree, this is the biggest problem.

As stated above, I think people who pull early are acting like idiots... but the way people choose to react is just as responsible (or moreso) for the sometimes-toxic atmospheres.

There are also times (quite often, in fact) when everyone waits patiently for more people to arrive before pulling. Even then, there are often people who start criticizing others in /shout for early pulling, even though the NM was definitely not pulled early.