The short answer is that it’s not all that useful to compare kernel versions between distributions; you should compare distributions as a whole.
KernelNewbies provides good summaries of the changes in each mainline kernel release. Here are some of the significant changes adding new features:
This doesn’t include improvements to the kernel itself, which are numerous, with performance improvements (and regressions), scalability improvements, security improvements (address space randomisation, sanitisation etc.), bug fixes; and support for new hardware. This is what most users would gain by upgrading to newer kernels; while the added features listed above are nice to have, most of them are only relevant when used with corresponding userspace tools, and distributions running older kernels without the necessary support won’t include the tools either.
Distributions really are best used as units: their kernel, along with their libraries, and the programs they package.
Comparing kernel versions also misses the backported features which some distributions include. Broadly speaking, distributions can be separated into two categories: fast-moving distributions which stick to the mainline kernels, and long-term distributions which maintain a stable base kernel for a very long time. Fedora is an example of the former, RHEL an example of the latter, and Debian falls somewhere in between, with mainline kernels tracking updates to a long-term stable kernel release train (4.19 currently, which is up to its 103rd stable release). So while the use of a 3.10 kernel in RHEL 7 might suggest that you’re missing out on lots of drivers, performance improvements, and new features, many of those are backported and available to RHEL 7 users.