This is a pretty long post, but if you want to just get the gist, you can read the bolded lines and the summary at the end.
Before I begin, I'll mention that I don't disagree with most of the statements that have been made in this thread or in others; however, I do take issue with some of the more pointed ideas. In this post, I will lay out what I think the standards for title formatting and minor edits ought to be.
1) Minor Edits -
Many edits are objective improvements to a post. For instance, they might correct spelling, grammar, and/or math. However, the sort of edit at issue here falls into the gray area of judgment calls: "Does this edit improve clarity? If so, by how much?" or "Does this edit help the post conform more closely to the expectations of our users?" As a result, we must all use our best judgment when making and reviewing such edits. As David Z points out, we must try to make edits that improve the experience not only for ourselves but for everyone. For this reason, I think that editorial restraint is the most important standard for minor edits. That is to say: "Even if a post might be slightly improved by my edit I should not make this edit if the post is good enough already."
Neither I nor anyone else knows what style or structure is most clear for the majority of users on this site because we are all - each of us - only one person. Therefore, just as we ought to try and put ourselves in others shoes when reviewing whether to keep an edit or not, we should also do that when deciding whether to make an edit in the first place. It is not our place to make sure that every post on this site is as good as it possibly can be, and even if it were, we have no way of knowing that the small stylistic change we are about to make will improve anyone's experience but our own. As a result, we ought to err on the side of the author when deciding whether a post is "good enough already."
2) Title Formatting - There are two slightly different standards for capitalizing titles at issue here. The first represents most written works and other works or art (for which I will follow David Z's lead and refer to it as "Title Case"). The second is a common but not entirely universal standard for titling academic papers (which I'll call "Sentence case"). I contend that both have a place among post tiles on this site and that there is no reason to apply one standard over the other in all circumstances.
It is my opinion that a good post title conveys the most important information about the post clearly and in the fewest words possible. Sometimes the best way to do this is with a short phrase consisting mostly of nouns - this way the topic is presented cleanly and directly. Sometimes the best way is to use a short sentence fragment usually starting with a participle (this is a style common among academic papers). And sometimes the best way to do this is to ask the question itself in the title; however, this is often not possible because the topic is either too complicated or not understood well-enough by the questioner to allow for this. The first type of title benefits from "Title Case" while the second two make more sense using "Sentence case."
Now let's address this specific example. "Spin Glass Prince Rupert's Drop" is a newly coined phrase that is the combination of two existing phrases: "spin glass" and "Prince Rupert's drop." Each word in this new phrase is equally important, and de-emphasizing any of them muddles its meaning. Of course I can only speak for myself, but when reading titles I tend to skim. I read all of the words but really only pay attention to the most important words, and I take my cues on what is important from the capitalization of the title. This is because I have an expectation that the author will both make sense with his words and convey meaningful information to me. I largely ignore lower-case words as essential to the sense of the title but not to the title's meaning. Therefore, changing the title to "Spin glass Prince Rupert's drop" is a detriment to the reader because he will instinctively take the title's meaning from "Spin Prince Rupert's" and be forced to focus more closely or read the title again to avoid missing a significant portion of the content.
At this point, I re-emphasize that this is only a marginal decrease in clarity. The title works either way, but it is for precisely that reason that it should not have been changed. Editorial restraint ought to be the standard for edits of this sort, and this edit demonstrates anything but.
3) The Uses of Different Titles - As I mentioned earlier, I think that both capitalization conventions have their place. "Title Case" is good for short titles that are conveying a large amount of meaning in a small space, while "Sentence case" is much more suitable to longer titles that are expressing more nuanced ideas. It would be silly to use "Title Case" in a question or a long sentence fragment because it would muddle the readers ability to parse the sentence's meaning.
For example, the title "Finding the electric field outside of a uniformly charged sphere" is vastly superior to "Electric Field outside of a Uniformly Charged Sphere" and slightly better than "How do I find the electric field outside of a uniformly charged sphere?" My reasoning is that the first title emphasizes the fact that the question is about computation while including the necessary information about what the computation will address. On the other hand, the second title is not only exhausting to read it is also vague - "What about the electric field?" the reader is forced to ask. Finally, the question form of that title is pretty good, but it is unnecessarily long and buries all of the meaning.
As a result, each of these types of titles is good for different sorts of questions. Short, "Title Case" titles are good for analogical or existential questions and toy problems. Sentence fragment titles are good for computational problems and problem solving. Question titles are good for succinct, direct questions (like, "What causes a double rainbow?" or "Is there a hottest possible temperature?"). But of course, this is just my opinion and other people might disagree with me. Someone might title a question "The Formation of Double Rainbows," and I probably wouldn't feel the need to edit it despite the fact that I think that it is a sub-optimal title.
It is for these reasons that I think that the original title of the post in question is still the best. "Spin Glass Prince Rupert's Drop" is short, direct, and puts all of the important information on display. "Spin glass Prince Rupert's drop" is pretty good but obfuscates the important information unnecessarily. And the post's current title "Is there a spin glass version of a Prince Rupert's Drop?" is astoundingly bad because it buries all of the important information and wastes a large number of words to do it. It is by no means necessary that all posts have a question for their title: this is a site specifically designed for people to ask questions; we know that they are going to ask a question in the post the title just needs to tell us what the question is going to be about.
Moreover, there is no pressing need for the title to be the whole post. A well-written post will present its question and the motivation for it clearly. Take, for example, the title post of this very thread. It does not follow some of the conventions that have been put forward by other users, but I doubt that most people would find it confusing or unclear. The title clearly states the issue, the first two sentences ask the specific questions, and the final sentence restates those questions in a different way. Titles are important, but the content and structure of posts are vastly more important.
4) Summary - In short, I feel that edits (especially those made by users who do not require approval) should not be made unless its contribution to the post is substantial. Likewise, reverting edits should follow the same rule of thumb. In other words, we should micromanage less and trust the judgment of users more. Additionally, I don't think that there is a compelling reason to enforce "Sentence case" over "Title Case" in post titles since they both have a place and can be used in different circumstances to varying effect.