Why do they hate John so much?
Josh Marshall's point that "the only serious opposition to McCain within Republican circles appears to be from people with active personality disorders (admittedly a non-trivial number of people, but a reality nonetheless)" is well-taken (Coulter is especially entertaining, as is the Freeper attack on Nancy Reagan), but I think it still doesn't satisfactorily answer the "Why?".
There are a number of reasons, of course, but to my mind the most important one is the torture issue. Over and over during Republican debates, McCain stood out in his opposition to the use of torture. He had credibility on the issue, too--not only because his stand was consistent with long-standing (albeit regularly ignored in the past) American ideals, but because of his first-hand experience as a prisoner of war.
It was, notably, a principled stand--perhaps the only principled position McCain has left to his name.
And it had the additional virtue of exposing advocates of torture as the macho poseurs that they are. No doubt this was especially galling to them because most torture advocates are at some level uncomfortable with the practice (shown by their eager embrace of such arrant nonsense as ticking bomb scenarios).
Furthermore, he held on to it under pressure. Pundit types who are used to jerking political strings don't like that. Even though McCain's self-humiliation in sucking up to the Bush League and the religious right in order to become more palatable to them was the stuff of legend. Even one principled stand is too many.
Contrast Mitt Romney, who seems to be the new darling. He couldn't fold faster if he were perforated. He has totally reinvented himself, dropping support for abortion rights and suddenly showing reverence for Saint Ron that was invisible in past years. No fears there that he will stand up to pressure on any issue (although if this is their thinking, I suspect they are making a mistake--pressure comes from many directions).
The irony is that McCain has tried to do the same thing, going so far as to promise to oppose his own legislation (especially ironic because he did have an extremely conservative voting record and is probably the most enthusiastic supporter of idiot military adventures in the party) but that is what went down the memory hole, while Romney's past is conveniently forgotten.
So what happens when, as seems likely, McCain becomes the nominee? My guess is that he will say or do something that can be spun as repentance, and the current opposition gets flushed (in John Holbo's wonderful words, "We have always been at peace with McCainia").
Meanwhile, it's entertaining, to be sure.