One of the most common accusations against anti-fans is that anti-fans are jealous of SNSD. Naturally, this is a common and almost legitimate argument to fall back on because SNSD is from the same company as DBSK and Super Junior, both of which have a very large female fan base. Furthermore, regardless of any plastic surgery, the SNSD girls aren't really that ugly in my opinion, although beauty is in the eye of the beholder (for example, I think many of the users here would disagree with me ^^).
Therefore, defensive SNSD fans feel that they have a sound argument when they accuse DBSK and Suju fans of hating SNSD just because "they are close to the oppas that you wanna marry." Of course, this incenses the situation even further and is kind of a low blow; I am a girl and I think that one of the things all girls hate to be called is desperate.
But the matter of the fact is, most of the people who know about SNSD--the very root of their fanbase, right when they first started out--probably found out about SNSD because they were already aware of SM Entertainment, DBSK, Super Junior, and other class acts like Zhang Liyin and The Grace. Even if they were not a fan of any of these groups, per se, they were probably aware of them because of all the hype there is about them in Kpop--who can ignore the rabidly screaming fans during an artist's performance and not wonder how they'd garnered such fervent admiration; it would probably deserve a Wikipedia or a Youtube search at least--and from there, their eyes would probably perk up when they spotted a piece of news about SM Entertainment's newest group, their newest project...
So, SNSD does owe a good part of their fame to their seniors and their fans, who kept the buzz up (unintentionally, perhaps; for example, my two friends who are really into kpop boybands (DBSK, F.T. Island, Big Bang) discuss it together all the time, and one day they were talking about SM and the new debut girl group, and who was sitting alongside them but kiddy old me, guffawing, "9 members?? for serious??"...and then I went home and googled), and while I'm sure it is true that if a DBSK member showed up on any random fangirl's doorstep (the rabid kind, the shy kind, the interested-but-only-check-'em-out-every-once-in-a-while-kind; it all goes) and proposed marriage, they would probably accept on the spot, no dates necessary. If not for the dude's singing chops and personality, then because he is famous, and I'm sorry to say that girls, but we like a romp and a bit of glamour now and then, huh?
But despite all of this, at the core, Anti-Fans are not jealous. Most of us have resigned ourselves to the fact that we are never going to marry or date these people; it would be nice just to see them perform in person. If we do harbor those kinds of romantic dreams, well, they're dreams, we understand--you don't see anybody declaring "Hero Jaejoong <333 I'm going to meet him and he's going to fall madly in love with me in the .03 seconds he sees me at the airport" except as a joke. This is common to all singers' fandoms, even SNSD; they're singing love songs for Heaven's sake, usually in the second person, don't use the premeditated effect as an insult.
Instead, SNSD's closeness to popular boy groups and scandals like Taesu and Jessica/Donghae probably create the impetus for fans of those groups to go online, find out more about SNSD, judge for themselves whether the rumors are true, and see and learn about the person whom everyone thought could possibly be so-and-so idol's girlfriend. I think intense fans in particular would take the news with a grain of salt; they would not want to believe that their idol is attached. But with the expectations they place on their idol's girlfriend, they would tend to be skeptical of whoever that person is and would probably expect them to be pretty above average (so yes, there are some relationship dynamics involved, I concede that, but not to the extent where "antifans suck because they're really just jealous bitches" is necessary).
And in encountering SNSD...well, it'd probably induce some head-scratching. Not because they're ugly or anything (again, what I think), but because of their songs and their image, which are kind of contradictory.
The girls are singing about meeting someone "at the end of my wandering", "don't tease me 'cause I'm young", "kissing you baby", all really sweet stuff, a bit like DBSK "Hug" and SuJu stuff like "Happiness", but SNSD is different because, erm, that's kind of all they do. SuJu has sexier stuff like "Don't Don" and DBSK songs now include lyrics like "I really wanna touch myself". *cough cough* Also, SNSD's style--short skirts and what I really can't stomach, bare midriffs, no pun intended--are inserted amidst all this syrup, and it just comes off looking skewed. The subliminal messaging is almost lewdly blatant to teenage girls, maybe not to younger girls and fanboys, but definitely to teenage girls, who are at the point in their lives when they want to be noticed, and not just in a "cute" way either, except that they can't be too obvious about it or look like they're trying too hard because society disfavors female sexual overtures...but SNSD's flavor kind of circumvents the rules. Their image is definitely fresh and innocent, but coupled with the skin they are showing, it's meant to be kind of sexy too, and in real life it just doesn't work that way. Teenage girls understand that you can't have both, hence the "SLUT!!" parade.
To many teenage girls, the SNSD girls just don't seem like potential friends. With their image, they don't seem to have the same desires or wants, and coupled with the fact that they are earning a living while doing what they love--if you want to make an accusation of jealousy, this is the right reason--and that their individual personalities aren't easily discernible (see Park Kyung Lim Wonderful Outing and UFO Interview to glean the girls being shy, awkward, and confident at the wrong times; also, there are just so many of them!) and often relatable to in the way that "Oh, I act like that when I want to impress someone, yup", which they can do now with more assured ease because these are girls and not boys, and then all the bad rumors there exist about SNSD, many of which haven't been cleared up in a cohesive article by the fans if they aren't true, and the fact that thousands and thousands of fans actually got it together long enough to boycott them with silence (the logistics of it!), and Schadenfreude, well, they aren't going to get much sympathy from the largest fanbase of all, young teenage girls.
They're not jealous. SNSD is just not friend material, and it's okay not to be friends, but if a person's personality is not just neutral and boring but also overbearing* and rude**, then that person's status elevates from being "not a friend" to being that person whom people dread spending time with and talk about behind his/her back.
Case in point: Wonder Girls' larger female fanbase.
Wonder Girls were chosen by both girls and boys as an idol group that they would want to usher in the New Year with. [source] SNSD was chosen by the boys only. (WG was also apparently first among the boys, SNSD came in a close second.)
Wonder Girls' image is pretty straightforward. "Irony" was supposed to be sexy, "Tell Me" funny, and "So Hot" both sexy and funny. There are no midriffs showing in "So Hot." It's a subtle sexy, and it's something that both boys and girls can laugh at--"Tell Me", too, with So Hee acting as Wonder Woman and that flasher and all the girls staring in fawning admiration at So Hee when she saves them. The lyrics match too.
Meanwhile, SNSD MVs are usually filled with closeups of the girls doing cute things, like running around under a mushroom in Alice costumes or popping up with cute expressions (go to 1:24 and 1:26 in the Girls Generation MV where Yuri does some truly weird things). Except in Into the New World, there's not much of a coherent story.
*In UFO interview (Part 4/05 on youtube, at around 1:20), Jessica answers the question, "Do you think you are all angels?" with "Yes, angels, because we are all pretty and kind." Granted, that entire show was embarassing questions, but I reckon Jessica could have left the first part out. Then, on arirang radio interview, Tiffany said that "SNSD has 9 beautiful girls" when giving a description of their group. I think "talented" or something would have been a more fitting thing to say, but they decide to focus on looks. (Come on, if your classmate was the star of the school play and was trying to get you to come watch, would a reason she would give be, "Because my costars and I are gorgeous"? That may be the case with movies starring Angelina Jolie and stuff, but it's not like you go around and proclaim that--I can tell that Angelina Jolie is hot stuff just by looking)
** Correcting Kim Ji Suk's English and the long list of stuff
on the "Why did it happen?"