I looked more into this on Wikipedia. (I'm posting this as a new answer instead of in the comments so more people see it.)
For reference, see the Wikipedia featured article criteria: https://en.wikipedia.org/wiki/Wikipedia:Featured_article_criteria
Obviously, the criteria do not translate perfectly from Wikipedia to WikiTree, but I think we can get inspiration from there. For example, profiles should be well-written, well-sourced, stable, include images where it makes sense, etc. If there's lots of text about a place, event, etc. it should be removed and split off into a Space page.
I agree with what Brian said, that sourcing should be the most important criterion. Nice looking profiles are great, but only if they are also reliable.
I like Chris's idea of maybe making this part of the Profile Improvement project. This is that project's area of interest, and would help to give group members something to focus on, which that project lacks in comparison to the other groups.