The details of the various EAPIs can be found in the package manager specification (PMS); the most notable new feature of EAPI 5, which has sped up acceptance a lot is the introduction of so-called subslots. A package A can specify a subslot, another package B that depends on it can specify that it needs to be rebuilt when the subslot of A changes. This leads to much more elegant solutions for many of the the link or installation path problems that revdep-rebuild, emerge @preserved-rebuild, or e.g. perl-cleaner try to solve... Another useful new feature in EAPI=5 is the masking of use-flags specifically for stable-marked ebuilds.
You can follow the adoption of EAPIs in the portage tree on an automatically updated graph page.