Also forgot to mention, the whole "Eldar love-fest" was more the creation of another way to deal with Slaanesh. It essentially boils down to the fact that all of Eldar society is essentially defined by the different ways they reacted to Slaanesh. The Craftworlders responded by shunning what they are, restricting their lives by the Path system, only allowing themselves to experience a fraction of their potential at a time, which does protect them by and large, but runs the risk of creating obsessions and being "trapped" on each Path. The Exodites followed a similar route, but more extremist, going back to nature so that they focussed only on survival.
The Dark Eldar, on the other hand, responded by refusing to admit there was a problem. They still follow the same lifestyles because, in their mind, the Eldar are the superior race in the galaxy, and reality is theirs, so why not have some fun with it? Sure, the mon-keigh might bleat about how they're in agony, or that "you can't do this", or some other pathetic whining in what they say counts as a "language", but really, they're just cattle. Less than, even. Of course, the Dark Eldar tend to ignore that Slaanesh drains their souls because of their link, and that's why they have to do what they do, and have to stay in Commorragh, but to the Commorrites, it's just a case of doing what they want, and the fact that the master does not live amongst his flock.
Now though, the Ynnari have come along, bringing a third option. Ynnead is a way the Eldar can actually fight back against Slaanesh, rather than merely existing (or ignoring) Her. Ynnead will claim their souls for itself, and will become strong enough that it can defeat Her. The issue is that, to the Craftworlders, this is basically heresy crossed with a conspiracy theory, as this is a God that hasn't existed, and still technically doesn't, and the strategy of using the Infinity Circuit to create an artificial afterlife has worked so far, so why change a good thing? Ynnead could just be a fools errand, and each soul that's taken from the Infinity Circuit, absorbed by these new "prophets", are souls that could be doomed to eternal torment.
To the Dark Eldar it's even worse, in a sense. Ynnead is proof that they could potentially be wrong. They've built their entire society around their current way of life, and now a cult has burst in saying "don't worry! We can save you from this! You can be free!" The Haemonculi are even more terrified, as their position in society is dependent on their usefulness, in both creating new Dark Eldar, and in reviving the dead. Now this other group has come along, demonstrating incredibly similar abilities, so they're fighting back against what the Covens see as rivals for their own power-base. Most of the Kabals/Cults just see the Ynnari as defeatist fools, as come on, the Dark Eldar need saving? They're having the time of their lives! They own the galaxy! Sure, Slaanesh wants to claim their souls, but what's life without a little risk!
Of course, a whole bunch of Eldar have joined up. Previously, their own myths and legends stated that Slaanesh would only be defeated at the End of Times, so they can finally be free of She Who Thirsts, but only after everything is destroyed. Now though, there's the option of another way. A way to bring forward the defeat of Slaanesh, without having to go through the Apocalypse to do so. To a lot of Eldar, this is worth the risks. Sure, they aren't protected by the Infinity Circuit, but as Malan'Tai and Iyanden shows, that idea isn't exactly foolproof anyway, and to a bunch of the Dark Eldar, it's a way to relieve the ennui of the ages, see something new, and get converted along the way. The Ynnari don't exactly say they can't keep acting the way they want afterwards, anyway. They just have to actually admit that Slaanesh impacts on their lives more than they want to say.
Then you've got the Harlequins, and who knows what those guys really want. A bunch threw their support behind the Ynnari, others decry it as heresy, others think that everybody needs to stop and worry about the Necrons instead. Basically, they're shifty.