I dunno, KDevelop 4, based on KDE's rather than Gnome's libraries, was nothing like KDevelop 3 and couldn't do some of the old things, and there was no way you could compile KDevelop 3 on an Ubuntu box bundled with KDevelop 4 and the new KDE libraries unless you were really good at this shit. Eclipse, based on the JVM, had a popular plugin that was broken, I think, in Juno or a bit earlier, and whose author refused to update it for the new Eclipse because too much of the things in Eclipse he depended on changed. Upgrading Emacs broke everyone's emacs-lisp hacks that they copied from each other. And so forth.

I think that in the land of Linux, the only things which remain backward compatible are the kernel (if you want to run statically linked userland binaries - not if you want to run driver binaries) and the GNU userspace utitilies like sh and grep (though sh is bash on some distros and dash on others, breaking the very popular assumption that sh is in fact bash, and env might reside in either /bin/env or /usr/bin/env, defeating its purpose of letting you deal with the fact that #!/bin/tcsh will break on systems keeping tcsh at /usr/bin. But a given distro will usually keep its shit where it last put it, which I guess is better than it could have been.)