I waited until after Yom Kippur to discuss this issue, in order to avoid needless controversy on the Eve of Yom Kippur.
By now, everyone who is interested is aware that a prominent Lithuanian authority has ruled that wearing crocs on Yom Kippur is halakhically permitted, but not adviseable. The reason, as reported in the Yeshiva World, is that the ordinance against wearing leather shoes is aimed at causing discomfort, and that the level of comfort afforded by crocs vitiates this requirement.
I find this ruling, במחילת כבוד תורותו, extremely difficult to understand.
First, I question whether it should have been published, since it was quickly transformed into an outright prohibition.
Second, I fail to understand the logic. The rabbis prohibited wearing leather shoes (נעילת הסנדל) as a way of fulfilling the Torah's requirement that we afflict ourselves on Yom Kippur. Anything not made out of leather is permitted. Comfort, prima facie, is a highly subjective thing. For example, I suffer every year by wearing sneakers or synthetic sandals (including Teva Naot). My family can attest that after the fast, I change into real shoes (or leather Naot sandals) right after Havdalah and announce what a pleasure it is. For me, crocs are an affliction. Other people feel differently. They say that cloth shoes, slippers, Teva sandals and crocs are wonderful and leather shoes are comfortable. Indeed, they wear them all the time. So what is it? Everyone decides what's permitted and what's prohibited for themselves? Such an approach is in violation of Hazal's explicit rule: אם כן, נתת דבריך לשיעורין, ie you shouldn't make your rulings based upon such subjective criteria. This, it seems to me, is especially the case when by all accounts crocs are absolutely permitted, even according to the authority in question. אתמהה!
One could respond that this is a matter of religious policy. I'm willing to grant that point, but there are other considerations. What if wearing more comfortable, non-leather shoes prevents a person from other admirable behavior such as standing through all of the davening, or even walking to shul for Yom Kippur. Does an ascetic stricture trump either of these? אתמהה!
Furthermore, why does this become a public pronouncement? If this was a suggestion offered to one enquirer, who says it was intended to become policy for the entire world? In addition, such declarations play into the hands of sectors who are only too happy to poke fun at the Torah. In the age of the internet, that should always be a primary consideration (and one of the reasons that I am firmly opposed to SMS and Internet responsa). Certainly, that was not the Posek's intention.
Finally, Yom Kippur is an exalted day. It is a sublimely spiritual day. Its halakhic perameters were determined by Hazal. What is prohibited is prohibited. What is permitted is permitted. As the Aharonim point out, over and over again in Yoreh Deah, הבו להו דלא להוסיף עלה! Don't push unnecessary strictures. The Torah demands sacrifice, no question. It also demands we be allowed, within the requirements of the Law, to properly worship our Creator. והמבין יבין. As R. Akiva Eger was wont to say: ה' יאיר עיני.
[UPDATE: I am grateful for the learned comments that this posting generated. Such is the way of respectful discussion in lehrnen.
In the interim, I have been informed that מו"ר Rav Herschel Schachter שליט"א reports in Nefesh HaRav (p. 210 no. 2) that מו"ר the Rav זצ"ל advised against wearing sneakers with arches, in accordance with the view of the Rambam that the footware allow one to feel the ground underneath him (which Rambam maintains is a Torah consideration). Once again, the same considerations made above hold, even in light of this testimony. 1) The overwhelming number of authorities disagree with this ruling, and thus there is no way that the Rav would've said that such sneakers are forbidden. He advised that one consider the Rambam, which would be typical of him. 2) There is no way of knowing how the Rav would've ruled in a world in which sneakers all have arches, or whether he would today have taken other factors into consideration . (In general, there is far too much necromancy at work in deducing the Rav's possible rulings in contemporary circumstances. See the aposite comments of R. Meir Lichtenstein, here.) 3) The fact is that crocs actually do let you feel the ground under you.
I, obviously, have no objection to people being strict with themselves. I have my own Yom Kippur humros, too (which are between me and my Maker). My objection is the categorical imposition of humros upon an entire population, especially when they bear in their wake such negative implications as those outlines here and in the comments.]