Since I recently gained 2K, and quickly thereafter 3k reputation here on MSE the review queues became accessible to me. Every now and then I check on them to see if there is something that needs reviewing.
When I notice a red icon I'm under the impression that there are a lot of reviews to be done, but every time I get disappointed when after 1 review the queue is empty again. So I decided to look for the logic behind the different states of the review-needed indicator, which led me to this announcement by Jon Ericson. In which there is stated that the default values for the red button are:
Queue Default ----- -------------- Close 20 Reopen 5 Low Quality 4 Suggested Edits 3 .... ...
Apparently this isn't the case, so what exactly causes the review-needed indicator to turn red the instant there is a review item?
Secondly, is it desirable that there is always a red flag, instead of the gray flag when there simply is something to review.
Regarding the suggestion that a filter might limit the amount of review tasks available:
There are no items for you to review, matching the filter "[discussion]"