Start with dark, rich wood (floors, tables, chairs, beams, railing...)
Then contrast it with a nice deep natural color like green. Add some expertly placed soft lighting, some artistic flourishes, and voila, you have an elegant eating establishment. Perfect for date night (as I keep hinting to my hubbie). 
image source"Wood Tavern, a place that expresses the spirit of its neighborhood with the prim, wood-trimmed outlines of a Craftsman house gone interior-designer mod. With a big-grained hickory floor and walls a light-to-dark gradient of forest-y green, it’s a room wrapped in the woodsy semiotics of masculine comfort, shined up with a little varnish of urbanity."
Of course, everyone has a different idea of what is a comfortable environment. It's all about the consumer. What works in Rockridge, CA would not have flown in my old stomping grounds of Hampden, an up-and-coming neighborhood in Baltimore, MD.
image source
image source"The Atomic Age décor of Rocket to Venus, with its sunburst mirror, robin's egg blue paint, metallic accents and pill-shaped windows, pays homage to the legendary Hampden space program, or so the story on the menu reads. Inside a plain building with simple signage lies a great place to drink or dine. Cheap but tasty food, reasonably-priced drinks and a low-key atmosphere make this a great hangout " (source)
So, what sort of restaurant atmosphere makes you feel comfortable? When you're looking for a neighborhood joint for date night, to meet up with friends, or just a place to sit back and watch the world go by while sipping your beverage of choice, where do you gravitate to?