A big problem with all wiki-based projects is that the contributors must achieve consensus. The problem is bigger on text-centered wikis where the main content is a product of the community. A lot of articles on Wikipedia got deleted (and are thus lost forever) due to obscure notability criteria. Passages in articles get deleted (and are then difficult to find) because the contributors can’t agree with them.
In one way, the problem is smaller on databases of binary content like Wikimedia Commons or Danbooru, but it’s still there. The content itself is not subject to discussion and dispute here, but how it is categorized and tagged. The discussion is necessary. Any tagging system that becomes too complicated becomes nearly useless. If there are a hundred or more tags on one item, if there are tags that are very similar, it becomes more difficult to find something specific, not easier. Restriction in articles is mostly a choice, restriction in tags is a necessity.
This is where the Tumblr system comes in. The main database would be the usual community project, closely monitored to avoid double entries, or clearly mark them as such. Multiple reproductions of one painting may sometimes be useful, for example. Only the most basic, and undisputed, information would be in the main database. For paintings, the artist, the year, the museum where it is located. For portraits, the sitter. Each item might have a short description as well, but just the bare bones.
And then there would be the user galleries. Basically every registered user would have his or her own gallery, but there could be projects of smaller sub-communities as well, just as there are topical as well as personal pages on Facebook. Each of these galleries would have its completely own tagging system, just as every blog has. I see no necessity to restrict the number of these galleries, but it would be essential to rank them somehow.
Basically, whenever an item shows up in a user gallery, this gallery page is linked from the main description page of the item. There should at least be a sort of triage: Approved projects are listed first, topical galleries before mere personal (“my favorite pictures”) ones. There could be a system of “likes” as on Tumblr or Facebook. Besides, all the tags from the user galleries would be searchable, just as on Tumblr.
In the long run, I think such a project would be a lot more interesting than the pure community solutions à la Wikimedia Commons.