There are a lot of @todo and TODO annotations in the Mongoid codebase:

$ ack --nogroup '@todo|TODO:' | wc -l
      33

Some of them are quite old (see https://github.com/mongodb/mongoid/blame/master/lib/mongoid/fields/standard.rb#L77-L82).

It's probably not a high priority, but I think it's worth looking through them and deciding whether they can be removed ("we're never going to do that at this point") or creating tickets for them so they can be tracked properly.