If you have one formal and one casual dining space, you can really go to town with different chair designs for both. A casual spot will do best with sociable, comfy seating that naturally leads to conversation, whereas a formal setting is where you can play around with luxury fabrics. If you only have one dining room, you're going to need to combine all of these elements in one set of chairs, if possible!
Dining tables and chairs absolutely need to be proportional with each other, as there's no use in having a low table and tall chairs! For a pared back, organic and very zen installation, Japanese designs can be looked to for inspiration, with tatami mats housing a low table and numerous floor-mounted seats. With back supports in place, comfort AND relaxation will be combined to perfection.