Global/Prevalence/Redundancy/Interesting or Informative -- the four criteria by which we are to evaluate new category proposals in peer review. (The implication is that these are the criteria for creating a category in the first place).
I would like to make a plea for another criterion, whether explicitly stated or not. My contention is simply that each category should have objective criteria to apply when evaluating a submitted waymark for acceptance into the category.
Descriptive words such as, unusual, exceptional, beautiful, artistic, humorous or funny, unique (as commonly applied), odd, outstanding, curious, noteworthy, etc. are all subjective. There can be other criteria which are subjective that don't necessarily use such subjective words, but require a judgment call on the part of the reviewer which may be open to different interpretations.
Now, the disclaimer. I don't at all claim that we can, or should, remove all elements of subjectivity from the review process. That is why we have reviewers - to exercise judgment.
But, here is what happens when primarily subjective criteria are used.
A). Submitter thinks his/her waymark is funny, odd, artistic, unique or whatever, and the person who reviews it doesn't see it that way at all and rejects it. Maybe a group vote sees it that way.
B). One or more of the officers is very strict in applying the subjective criteria and declines a lot of waymarks.
C). One or more of the officers is very liberal in applying the same subjective criteria and accepts almost everything.
This results in confusion by those wanting to submit a waymark. What really is going to be accepted? There can be hard feelings between a waymark submitter and the reviewer who declines the waymark, and there can be internal conflict within the mangement group if the standards are being applied differently. Ultimately, the integrity of the category suffers from a lack of consistency.
It sometimes takes a lot more work to write a category so that it is objective in nature, but I think it is well worth the effort. And, there are probably some ideas that will always just be too subjective to be workable. Others can at least be revised to be more objective.
Defining terms can help.
Old - how old, exactly? Use years, dates.
Big - how big? Use measurements, at least approximations.
Using examples, as some categories have been doing, is an excellent help. It doesn't entirely solve the problem, but it gives the reasonable person an idea of what will be accepted and what will not be accepted.
Lists: Using available lists, directories, etc. can be another way if ensuring objectivity. There needs to be enough flexibility to allow for noteable exceptions in some cases, though. (See, some subjectivity can be good).

As a reviewer, I want to have these things to depend on, so that if I have to decline a waymark I can give a good reason and not have to say, "I don't think it is cool enough to accept," or something. And, as a waymark creator, I want to be pretty sure ahead of time that the time and effort I put into putting together a waymark isn't going to be wasted.
Last desclaimer - Although there are lots of examples that could be cited in both directions, this is not intended to single out any particular individual or category, living or dead.
Whew, I really needed to soapbox this issue! (Is that a verb, really?)