It's my fate in life to natter on about how difficult second books of series are. Can we forego all that and get to the meat of the problems?
Firefight has the same tropes as the first volume in the series (Steelheart), i.e., young nerdy kid, with an aptitude for fighting Epics and a super-creative mind, who has a heck of a time creating appropriate metaphors (nice to know we have one thing in common). Obviously, he's meant to save the world in the end, and I don't think there are any spoilers in that. Why else would Sanderson make him the main character? Also, you'll see what I mean at the end of this second book.
But I think perhaps he's a bit too heroic in this book. Sanderson does his best to show that some things do require hard work in order to make an impact in the world (life lesson #149), but we're talking about fighting people with superpowers. It's just all too easy for David! We have to keep our protagonist alive, of course, but also too many difficult scrapes he gets out of too easily. And the denouement at the end is really bloody obvious. I continue to keep in mind that I am not the primary audience (young adult), but still! Ugh and bother.
Oh, of course I'll read the final book. It is Sanderson, after all, who's a better writer than most fantasy authors out there.