What do the national debt, your old gym teacher, and your mother-in-law have in common? They're all formidable — that is, they inspire fear and respect thanks to their size, or special ability, or unusual qualities. (Or possibly all three in the case of your mother-in-law.)
What's interesting about formidable is that we often tend to use it about things that, while they may scare us, we can't help being pretty impressed by all the same. A formidable opponent is almost by definition a worthy one; a formidable challenge almost by definition one worth rising to. Not surprisingly, this word is derived from the Latin formidare, "to fear."