Like others have said, it ain't so much that walnut is "bad"... just that there are better choices. As far as the "on printed paper" heating value, walnut is about like cherry, birch, hackberry and elm, but walnut has some disadvantages those others do not. For one thing walnut has a lot of oil in it and needs to be seasoned longer before it will burn well. It also doesn't coal-up much and tends to leave a lot of ash behind. And, like others have said, it burns-up relatively fast so it ain't a good "overnight" wood...and the faster it burns the higher the percentage of heat lost up the flue. Basically you end up loading the firebox more often, use a little more of it and clean out ashes more often than if you burn, say, hackberry or cherry.
Still, I don't leave it lay to rot... I just got done cleaning-up along the north side of my yard, took down 8 or so walnuts that were making a mess in my grass. I didn't spend any time on the smaller stuff, but I piled up quite a few 8 foot logs from 6/8-inch up to 24-inch diameter (still have one left to take down, the largest of the bunch). I'll get it bucked and split at my leisure, burn it mixed with the elm and such next fall/early winter... or maybe the season after that. I'd much rather burn walnut than, say, Box Elder, Willow, Cottenwood or Basswood... and lots of guys burn that stuff.
