Most SELinux policies, Corosync and Pacemaker and related HA / clustering tools, and a pile of other things and they settled for a really awful stable kernel major in Jessie / 8.x.
It really went downhill with, the release of 8.0, the only improvement we really saw was a slightly newer kernel (but still old) and SystemD (although not consistantly implemented), The Debian community has also fallen through the floor, lots of smart-ass bickering and slack package maintainership and generally unhelpful, rude contributors / owners. By contrast RHEL/CentOS sped up their release cycles after re-in-housing Fedora as and taking a lot of it's rolling updates into stable a lot quicker and the community has been fantastic, Redhat have really stepped up and modernised their game a lot over the past two or there years.