I am SO psyched about this ripple-along. I've gathered stash yarn, eliminating a few balls because they're too light or too heavy, and one that was just the wrong colour. That leaves about fifteen balls of DK to light worsted weight yarn to start, and I'll see where it goes. I'm using a 5mm hook and a wavy ripple adapted from the Harmony Guide (I'm working through both loops instead of the blo called for in the pattern - I don't want ribs). My two constraints beyond yarn weight are colour (though I won't be too picky) and natural fibre. The yarns I'm starting out with aren't all 100% natural, but I won't use any entirely synthetic yarns. This is both because I'll have to hand-wash the blanket anyway, so I might as well make it as soft and luscious as possible, and also because I love natural fibers and I want to love making every stitch of this enormous project. Oh, and also, I often can't resist buying a ball of delicious yarn to play with, and using it in the afghan will both ensure that I play with it and also that I use up all of it. Tonight: I'll make a long chain!
ETA: How wide are you making your afghan, and how did you decide?