Philosophical and Doctrinal Differences between Theravada and Zen, and its effects
The difference is an entirely different canon!  Theravada follows the Pali canon whereas Zen pulls most of its philosophical underpinnings from the Mahayana sutras and its own oral and written traditions (i.e. koan collections, writings of folks like Dogen and Hakuin, etc.).  That being said, I don't know how you would begin to explore what the effect on mindset would be.  I would also wonder if it even makes sense to make that investigation.  So much depends on the practitioner and their own development along the path.  I've known practitioners of Zen who were total, intellectualizing jerks and Theravadans who were oceans of compassion.

Outside of the obvious doctrinal differences, another rather ironic difference between the two camps is the practice of meditation.  Jhanas, as described by the Buddha in the Pali canon, don't often make an appearance in Zen.  I say ironic as the word Zen itself is a twice removed transliteration of the word jhana (i.e. jhana/dhyana -> ch'an -> zen).  In practice, Zen does not emphasize or pay much attention to being in, say, the third jhana over the second.    

I'm a little taken aback by your statement that Theravada Buddhism doesn't emphasize compassion to the degree Zen does.  If anything, the opposite is true.  You find all sorts of meditations on the cultivation of compassion and loving-kindness in the elder tradition (the Brahmaviharas for instance) whereas Zen doesn't take the same practical, directed approach to its development as found in the Pali canon.

Lasty, emptiness does factor into Theravada.  See the Maha and cula sunnata sutta for just two, albeit less emphatic, explorations of emptiness. 

And to reemphasize, so much of this depends on the practitioner.  I don't think it's helpful to view either school as monolithic.  Setting one against the other isn't necessarily helpful or, in my opinion, even possible.