Enterprise linux distributions pin package versions through the entire "major" release. They get security backports and often feature backports (for the first few years) but nothing which would change an API or ABI - for 10+ years.

As such, the answer to your question is "pretty much everything". Wildly different kernel, libraries, etc.

In theory it would be nice to roll upgrades like you suggest, but there are huge swathes of industry for which it's not a practical solution.