Just because dried mushroom stock is easy doesn't mean it's unimpressive. And the character of the stock varies dramatically with the character of the mushrooms you use. I'll use the shaggy parasols (pictured above) we collected with Dougal and Deidre to make a stock for a mushroom soup. Sure, you don't have to use a stock for mushroom soup, but if you do, its flavours will be more layered and complex.
You can use dried porcini to make very rich stocks for soups and risottos, and dried shitake mushrooms to make stocks for japanese style broths. Porcini and shitake are the most common mushrooms used for stocks, probably because they're both very flavourful. But I'm sure there are others you could try, once you get adventurous.
Making Porcini Stock
Making stock from porcini requires different techniques to making stock from shitake. Paula Wolfert recommends soaking 1/2 cup dried porcini with a pinch of sugar or salt (to draw the flavour out, I think) in 1 cup warm water overnight.
The next day, drain the porcini by lifting them out of the liquid without stirring up any sediment that might have settled, gently press them dry and chop them finely. Sieve the remaining liquid to remove any grit, preferably through a coffee filter. Recombine the solid and liquid parts if you like, or use them separately (you can, for instance, sweat the chopped porcini with onions at the beginning of a soup or risotto).
What you now have is a very concentrated stock that you can weaken with filtered water to taste. It's always best to make stocks without the ingredients swimming in excess water. If they do swim, the stock remains insipid regardless of how you try to concentrate it later.
If you forget to soak the porcini the night before, you can soak the mushrooms in near boiling water 30 minutes before you need them, but the flavour will be a little coarser. Porcini like to be treated gently.
Shitake, on the other hand, do well with a good simmering, and are one of three ingredients that when combined, make the classic dashi broth. But I'll save this for my next post.