I think Gale and Astarion are the two easy ones to miss, Karlach (not recruitable at present) is also out of the way, though not too out of the way. Gale is out of the way and Astarion is really out of the way. Wyll is less easy to miss and more easy to let die, I watched a bugbear crush his head in once, if you just watch the battle play out in stealth. Considering Shadowheart has an interesting story route if not recruited I would imagine she's a better one to miss/not recruit.
