I've been going through Eric Warner's book again recently, and I came across something I've not heard other homebrewers talk about. He says that off-flavors from fermentation byproducts can be the result of too much, too little, or the wrong amount of amino acids in the wort. It's not feasible to evaluate this on a home scale, or even a small commercial scale. Formol titration isn't capital intensive, but it only gives you total nitrogen, not composition, IIRC.
Still, it's made me wonder about people who use step mashes when maybe they shouldn't, and if that could cause low-level off-flavors in the finished beer.