mbuckner, as you might expect, most of the lower fare classes' prices and restrictions are determined by each individual airline. There really is no point in trying to compare a fare inventory from one airline to another, i.e., "L" might be the lowest fare inventory for DL but this does not mean that this is the lowest fare inventory for all the other carriers (BTW, K fares on DL may be significantly higher than L fares on most markets).
The fare inventory is similar for all airlines for full coach fares (well, most of the time; different airlines have their own designations for full fares like Y06, Y26, YUP, YX, etc.), business (C, J), and first (F, A).