Ignoring shape and pressing you want a blade that will offer you consistently high performance. Evenly spaced grains offer this, the more grains the stiffer the face but when you get a really tight blade 16+ the stiffness can make it hard to press so 10-12 would be the most common in the very best performers, colour does not seem to effect this even though the heartwood is a bit drier and therefore a little more stiff and brittle than the sapwood.
Speck and fleck is the depositing of minerals in the willow whilst growing, it tends not to offer any performance gain or loss but overall adds a little in weight.
Pin knots are harder and less responsive but can work well in the toe or shoulder of the bat.
Stain can come in many forms, the typical butterfly and bar stain are due to the willow coming from an older variety of willow, it was phased out for cosmetic reasons but those old enough to remember when it was common point to it offering a slightly lower density and improved resilience. Blue willow is caused by uptake of Iron, again this tends to be rare as caused by the siting of the tree's and adds strength to the blade.
Density is an unknown factor in growing, some trees produce low density willow but rather than being down to one factor or another is probably due to a combination and has proved fickle to narrow down. Low density tends to be found across the grades.
The end result is that when you look at the bats supplied by bat makers to players, and by that I mean professional players you see what we on the forum would class as pretty bats but also big for their weight.
For us a bat with clean straight and even grains is going to play better than one without, based on being made the same way.