While I don't think we should completely remove comparative-review, I do think it might be a good idea to considerably narrow the scope on what's allowed under this tag.
Comparative reviews should be written in the same language.
While it's not impossible to be sitting on the fence about which language to write your code in, if you're at this point, you're not ready for CodeReview. Moreover, if you're posting this question, you're sort of really asking, "which language is better, A or B?" and I don't think this is really all that great of a question for Code Review. Even if you're writing an Objective-C/Swift application in which the languages can be interchanged on a file-by-file basis, you still need to be decided on which language you're writing in before you're actually ready for a review. The choice of which language to use is outside the scope of Code Review.
Comparative reviews should not be performance or optimization reviews.
If you're trying to optimize execution speed, determine which of your two snippets runs faster and post that snippet by itself as a speed-optimization question.
If you're trying to minimize your applications memory footprint, determine which of your two snippets has a smaller memory footprint and post that snippet by itself as a memory-optimization question.
If you don't know how to determine which is better from a well-defined metric based standpoint, then search StackOverflow for how to measure whatever metric you're trying to compare. There are answered questions on how to make this comparison in basically every language.
Comparative reviews should not be too broad.
While it's okay to post relatively broad questions on Code Review along the lines of "This is my implementation of [x]. What can be improved regarding readability? Am I following best practices?", a question this broad does not work that well for a comparative-review question.
What, specifically, are you looking to compare between the two snippets? What do you believe to be the pros and cons of the two snippets, comparatively? And what, specifically, are you looking for in terms of determining which snippet is better?
Give reviews some idea of the criteria they should use in order to recommend to you the better snippet.
Comparative reviews might be better asked as two separate questions.
There's nothing preventing you from asking two questions.
If you're still trying to decide on a language, write one implementation in language A and post that question, and language A experts will answer with any and all improvements they see that can be made. While that question is collecting answers, write and implement the same code in language B and post that as its own question. Language B experts will answer that question with any and all improvements. Collect all the improvements for both implementations, implement them all, and make the comparisons for yourself--this may help you decide on a language.
If you're trying to create an optimized implementation, post both of your current implementations as starting points. See what sort of optimization answers each question collects. Implement these optimizations, then compare the updated versions of the code yourself.
Even a comparative review such as "which snippet is the cleanest" is still probably better served as two individual questions. A comparative review splits the focus. Is it likely that either snippet is perfectly clean? No. So both snippets can be served with some clean up. The answer to the question of "which is the cleanest" should be fairly obvious... the cleanest one is probably the one that got less scrutiny when you posted it as an individual question. And the upside is, both snippets received some clean up!
If you're looking for a broad, all-encompassing review of a piece of code, post it by itself. If you're looking for very specific comparison about specific aspects of two concrete alternative implementations, then maybe a comparitive-review is right for you.
So, what would make for a good comparative review?
Java method - levels of abstraction
I think this question serves as an example of what could make for a good comparative-review question.
It meets the first criteria described in this post. Both implementations are written in the same language (and do the same thing). Any language expert looking at this question could equally judge both implementations.
It meets the second criteria described in this post. The question is not about which is faster, which is more memory efficient, etc. If this were a question, the asker has already tested and determined the difference is insignificant.
It meets the third criteria described in this post. The criteria by which reviewers should judge the compared snippets is quite clear. In this specific example, we're asked to compare the level of abstraction of the two snippets. Is A not-abstracted enough? Or is B too abstracted?
Importantly, this review might not make very much sense if each snippet were separated into different questions.
The user could've posted snippet A as "Is this abstract enough? How can I make it more abstract?" and snippet B as "Is this too abstract? How can I make it less abstract?" but then the answers will focus as much or more on how to make the snippets more/less abstract and less on how to decide on the right balance. It would be essentially impossible for the user to receive the same answer on both questions if he posted the snippets separately! In fact, it seems likely that question A is the answer to question B and question B is the answer to question A!
With any sort of a performance comparison, is anyone actually trying to strike a balance? No. What the asker actually wants is the most optimally performing snippet of code. If the user presents snippet A and snippet B, the answer to the question might be to implement snippet C, which the asker never even considered. So, rather than an overly complicated question which first asks which is more optimal between two less-than-C-optimal versions of the code, in which any answer must at a minimum feel obligated to spend time discussing the performance difference between A and B, wouldn't it be better if we just expected askers to come knowing what of all their attempts has the most optimal performance, and instead ask how to make it more optimal?
And with any sort of "which is better?" question, isn't this too broad? Isn't this not seeking a review? Without appropriate, specific guidelines, isn't this primarily opinion based (even moreso than an average Code Review question)? Isn't this potentially just seeking an explanation of the snippets?
This answer defines a very narrow spectrum off on-topic comparative-review questions. But I believe it defines a spectrum that will lead toward very high quality comparative-review questions, and should encourage others (who might have otherwise posted a comparative-review question) to do a little more research and ask better, more direct, more interesting non-comparative-review questions.