This has been covered in this excellent post from Jon Ericson.
The change was because of the many issues and complaints about the old system (including that the number was always incorrect)
Now a red indicator appears
- if any review queue has a red dot (explained below) and
- if it’s been at least an hour since you clicked on the review icon.
So the idea is that when you see the indicator light up, it’s a reminder that there are reviews to be done if you haven’t looked recently. The goal is to draw reviewers’ attention to queues that can use some help.
Each queue may be in one of three states:
- No dot (empty queue)
- Grey dot (at least one but fewer than N review tasks)
- Red dot (N or more review tasks)
Each review queue has its own threshold (N). The default values are:
Low Quality 4
Suggested Edits 3
First Posts 10
Late Answers 6
So if you don't see a red dot, there may be review items, but they haven't hit the threshold. This may change your approach to reviewing (I don't know if for better or worse) but it is an SE-wide decision.