This regression is resolved in 4.1.0 RC 2. Old ratings will become visible and new ratings can be made.
The following content types can receive ratings: page edits, upload versions, feed entries, discussions, and special pages. For these content types, the rating score is computed based on a straight average of the most recent 10000 ratings a content has received.
The following content types cannot receive ratings directly, but have a rating score that is computed based on another formula: pages, feeds, and attachments.
For pages, the rating score is computed based on the scores of the most recent 10 edits, with the most recent edits weighted more heavily. 10 edits are used as a cutoff, because older edits will tend to be weighted so low that the performance trade-off is not worth it. A benefit of this from the admin perspective, is that editors who care about their ratings will have to be more mindful than before -- making small or repeated edits, which might bump their content spammily in feeds or watch-lists, could also impact their ratings. Still, in case no recent edits have been rated yet, scores are cached until new ratings are received, so it would be unlikely that the score will disappear just due to none of the recent 10 being rated.
For attachments, the rating score is computed based on the scores of the most recent 10 upload versions, weighted similarly to the page-formula in the last paragraph. However, attachments also include the scores of recent edits of their text content, but these scores are weighted significantly less than those related to the file contents.
Old ratings from prior versions are automatically converted to apply to an edit/upload based on the datestamp when the rating was made.
For both pages and attachments, attempting to rate them directly will actually rated whatever their current edit/upload is.
For feeds, the rating score is computed based on the scores of the most recent 10 feed entries. This is not the same as the score of the content that is used as an entry.
You can now leave a reason for your ratings.
Just in case of rating spam or rating vandalism (poorly or highly rating content for revenge or self-promotion), ratings can be reported and deleted by moderators.
There is now a view that shows all ratings for a content.
There are 3 new Special pages: Pages with the Most Ratings, Highly-Rated Pages, and Lowest-Rated Pages.