For those that don't have Palmers book, go get it! In the meantime, the higher numbers on the crystal grains roughly translates to stronger caramelized flavors, coming from longer kiln times.
The lower numbers from 10L to 40L may be sweeter, toffee and honey like, and the higher the number up to 120L and 140L gets increasing dark, raisiny, dark fruits, rummy, and what have you. Briess has the full lineup from 10L to 120 or 140L, whereas most Euro maltsters have four or five, spread more widely, such as Caravienne (21L) to Special B (147L).
I personally think the Euro's have substantially more flavor, I think because the base grain used is Maris Otter or their equivalent. They're noticeably bigger and more plump.