Stock should be simmered in an uncovered pan, is the conventional wisdom. Covered, the liquid boils too rapidly; impurities that would otherwise float to the surface, to be skimmed off, get absorbed, and turn it cloudy. A temperature just below boiling point is sufficient to extract the flavour and the collagen (which converts to gelatine).
Some years ago, I read a piece by Heston Blumenthal recommending that you cook stock in a pressure cooker. That way, he said, the flavour did not disperse. I was sceptical: was steam from a simmering pot really carrying away valuable flavour? Surely, if you wanted to concentrate flavour in a sauce, you boiled it uncovered? Nevertheless, I tried the Blumenthal technique. It produced stock that tasted flat and stale. (As I mentioned in the comments section of a previous piece.)
I wonder now whether the tired flavour was produced by overcooked vegetables. As I have written before, you should add vegetables to a stock no more than 40 minutes before the end of cooking it. And do we home cooks need to worry about cloudy stocks? We're not producing consomme - or, if we are, we should probable clarify the liquid anyway.
So now I am experimenting with covered pans. The advantage of them is that, once you are sure that the liquid is simmering gently (I use the smallest ring on the hob, and put a heat disperser under the pan), you can leave the pot for several hours without worrying that the stock is boiling away. The appearance of the stock is little different, and the flavour is at least as good.
Perhaps my next experiment should be to try the pressure cooker again, without vegetables.