Not that stability is always the right choice, I wish C++0x would have been more radical, for example. It does matter for long-lived codebases though.
The breakage in 1.8.6/1.8.7 was widely regarded as a massive failure on the maintainer side - a bugfix for a security vulnerability that had other changes land in the release (and subsequently segfaulted, sigh). That's however not a design change. Something like that has never happened ever since.
I actually agree with you up to a certain point: The ruby ecosystem is was and still is pretty unstable. Things have settled a little lately, but gems still appear and disappear, are superseeded by newer versions or other gems that do things differently. That's good in some ways, since stuff evolves and improves, bad in other ways - the lib you depend on just disappears or falls out of maintenance. However, none of this is the core-languages fault. Behavior of ruby as a language has been pretty stable during the last couple of years.