I actually think the Lightning character is pretty interesting/cool, and though she does have her monotone period (much like Cloud & Squall both) she has a fair amount interesting going on. The problem is the story she's in is all over the place, has no recognizable/significant antagonist (it's just sort of nebulously 'the gods') and she shares the screen with a few really unbearable characters, like Snow.
I personally actually think of Lightning as a character as one of the better FF protagonists, though, there's just not much good to use that character in.