I am noticing that in several of my roasts that the espresso stream is a bit thin, and the crema shows various deficiencies, ranging from small, visible bubbles throughout, to light color, and thinness. I am suspecting that my drying time may be insufficient. Are there any visible signs of roast defects in the cup?

Great question! Probably you could find some roast feedback in the crema, but the product in the cup is an equation with all of the variables of the coffee (i.e. how it is grown and processed, stored, roasted, ground, and extracted) all rolled up.

So I think to answer this question, you have to have a very systematic way of controlling the other variables that influence crema and shot quality.

I would guess that noone yet has that level of super-barista control. Standard cupping is probably the best way to evaluate your roasts since the espresso extraction process is so fickle w/r to temps, pressures, grind, tamp, etc.

That said... I do get fairly consistent extractions, and I do not see a significant crema difference from roast to roast. With each new roast, I have to tweak the grinder setting. So possibly your crema differences are related to grind setting more than the roast?

I agree with Don; there's too many possible causes for bad crema.