Most, because Debian is the only distro which strictly enforces their manpages and filesystem standards. And most source packages don't care much, resp. have other ideas

Lots. Because many upstream projects don't have their build system set up to work within a distribution (to get dependencies form the system and to install to standard places). All distros must patch things to get them to work.

A fair few I expect, amongst actively developed apps/utils/libs. Away from sid (unstable) Debian packages are often a bit behind upstream but still supported, so security fixes are often back-ported if the upstream project isn't also maintaining older releases that happen to match the version(s) in testing/stable/oldstable.