A main issue I see here is one of (perceived) consistency or fairness. Moderation on an SE site can never be an exact science, but it should not show too much variance either.
Personally, and those that know me from MO might confirm this, I am not that big a fan of big lists. However, they are popular, and good and interesting "lists" likely should have their place on this site (and they have shown on other sites to be in certain ways valuable). Indeed, there are already several such list on this site.
To me this one seems at least if not more focused and specialized than some other lists we already have. It is not very clear why this one should be closed while others stay open.
Added: Of course the standard's of a site can and should evolve over time, either smoothly and organically or when some (new) concensus or absent this at least modus vivendi is found on some issue. I thus certainly do not want to say: since we had one list we need to allow all lists. Only, it seems to me that here the change was a bit quick and/or not announced (or at least perceived as such).
Added 2: Upon request, some list-like questions.
These ask for certain books/written accounts, I would say pretty much like the one we discuss (and when pressed I would rather say less than more specific).
What are some great books for exploring mathematics? (not kids' books and not textbooks)
What are some great books for inspiring children to explore mathematics?
Books/(auto)biographies/references on how mathematicians study/studied (as students)?
This one asks for various presentations (note, it was controversial, but not mainly since it was a list).
Showcase of Powerpoint / Keynote / Beamer Presentations
Requests for poblems, examples, and so on, rather list like (somewhat different, but quite possibly would be CWed tagged big-list elsewhere).
Good, simple examples of induction?
Impressive examples where a "proof by picture" goes wrong
Mathematical problems for preschoolers
Finally, let me add there was a meta question
Books and materials recommendations
that allows certain requests for books and related resources. (Disclaimer: I gave the positive answer, but there was not much though a little opposition either).
Afterthought: It is in my opinion unreasonable to discourage such list questions too much early on. People like to ask about and share favorite things. And, this is not because they are used to CW and lists from math.SE or MO; to wit, Sue VanHattum is new to the SE thing and asked two lists. Also, having such questions around can also encourage new users to participate, really new ones not from the network, being a low entrance point. Maybe some SE veterans will raise an eyebrow seeing some lists, but I doubt this will ultimately affect if they join or not.
And as long as the site is somewhat small it is not really a problem to have such things; it starts to be a problem if the lists create too much "noise" and/or grow that large that they get unusable. If/when the site grows we can and will have to get stricter. But being overly strict at the start is not a good idea. Obviously, some standards need to be preserved, but too much
Opinion? No! List? No! Discussion? No!
will alienate too many people not yet used to it.