Radiant heat is an odd but cool thing. Think of it this way. Where ever you are, everything that is in line of sight to you is either radiating heat toward you, or sucking it away from you. The thing that determines which of those it is is temperature. If the object is hotter than your skin temperature, your skin absorbs the radiant heat. If the object is colder than your skin temperature, you radiate heat toward it. Your body is comfortable when your incoming heat is close to or equals the heat you're giving off.
So, that being said, what you want with radiant heating is large surface areas to be heated/cooled so more of that line of sight to your body is getting heat vs loosing heat. So, only using wall heating/cooling will definitely work, and might work great for certain places, but is not a great idea to do a whole building in IMO. Normally, a floor gives a great, flat area to heat up and gives great line of sight to your body. I would think you would need to do more wall heating than floor heating to achieve the same effect. Also, you'd probably have to heat exterior walls which would tend to loose heat to the outside faster than a heated floor.
Please keep in mind I am nowhere near an expert. I haven't even finished my book on modern hydronic systems yet.
