To paraphrase my mother, it's all fun and games until someone starts writing subclasses.

~ Russ Olsen

When the hierarchy is correct, anyone can successfully create a new subclass.

~ Sandi Metz 

OO languages make abstraction easy — perhaps too easy. They encourage architectures with thick glue and elaborate layers. This can be good when the problem domain is truly complex and demands a lot of abstraction, but it can backfire badly if coders end up doing simple things in complex ways just because they can.

~ Eric Steven Raymond

OO is about Messaging

The big idea is "messaging" -- that is what the kernal of Smalltalk/Squeak is
all about (and it's something that was never quite completed in our Xerox PARC phase).
The Japanese have a small word -- ma -- for "that which is in between" -- perhaps
the nearest English equivalent is "interstitial".

The key in making great and growable systems is much more to design
how its modules communicate rather than what their internal properties
 and behaviors should be.
Think of the internet -- to live, it (a) has to allow many different
kinds of ideas and realizations that are beyond any single standard
and (b) to allow varying degrees of safe interoperability between these ideas.

~ Alan Kay