As I read, ExifTool is tested with sample images of about 3000 camera models.
It would be very interesting to publish the exiftool reports of these models on the website. This allows exiftool users to know about the tag coverage of each model and trouble-shot model specific issues (i.e. no Exif ISO value).
Also some sort of accumulated statistic would be great (how many % of the cameras store serial numbers in the metadata etc).
The reports could be re-generated with every new exiftool production version with some sort of automated changelog (new tag discoveries, new models).