Thanks for starting such an interesting thread.   

If the "What is philosophy" is about what actually holds the set of practices together, might hunch is that it is less a set of subject matters (as has been pointed out), and more a lineage and specific history. On this view, answering "What is philosophy" boils down to an empirical, social historical question.  

I quite agree with the comment that one needn't define philosophy to do philosophy, but one place where struggling with the question is requisite is in an introductory level course -- i.e., when you're trying to explain to students the distinctiveness of philosophical content and methodology. For me this has a pragmatic, pedagogical payoff, as an entre into Plato's early dialogues and preoccupations with definition.  Of course  such an endeavor comes up short -- for interesting reasons -- of necessary and sufficient conditions.

I personally like to start with the etymology of the word, the "love of wisdom."  This makes philosophy the pursuit (not attainment) of some sort of unifying integrative understanding, animated by wonder, that gives us guidance in how to live well (= virtue). 

While consistent with the ancient sense of concept (see Pierre Hadot's work on this), this conception doesn't particularly fit our more specialized, disciplinary practices of today. 

To complement the etymological definition, I also add Simon Blackburn's very terse definition of "conceptual engineering," and Wilfred Sellars' "The aim of philosophy, abstractly formulated, is to understand how things in the broadest possible sense of the term hang together in the broadest possible sense of the term." (from his "Empiricism and the Philosophy of Mind." ).

My closing questions are: what place does the more ancient conception of philosophy have in our modern practice? Also: how is wonder different from curiosity? 
DH