I think the strongest correspondance (commonality) of Zen and Theravada is with the emphasis on seated meditation and mindfulness practice. One thing that's puzzled me however is that while Zen Buddhists teach and practice mindfulness extensively they are less likely to talk about it in as detailed a manner as Theravadins. Thich Nhat Hanh is the Zen teacher who has taught mindfulness most extensively, along with Gil Fronsdal. Both make use of Pali sources, so it's a rich area where there are strong similarities in method, as well as some differences.
I was initially drawn to Zen through Shunryu Suzuki's book Zen Mind, Beginner's Mind
, and experiences i had in Japan with the traditional arts, which teach right mindfulness, concentration and effort. That and Thich Nhat Hanh's book Miracle of Mindfulness
have had a strong impact on my practice. Now as i dig deeper into Theravadan teachings i'm drawn most to the IMS and Thai Forest teachers, and i think the reason there is that emphasis on being mindful moment-to-moment, with your body, breathe, movements and mind.
What is the difference between mindful walking (on a vipassana retreat) and mindfully making tea, cleaning a Zen temple or doing caligraphy? These are different exercises that seem to cultivate similar understandings and skills.
We may have different ways of talking about and conceptualizing the dharma/dhamma but for those whom meditation and mindfulness are central to their practice we are sharing a huge common ground.
In my opinion.