I'm at a loss when it comes to judging how magically powerful Lily herself really was. All we have is a lot of hearsay and one accidental act of magic that does not seem to necessarily be dependent upon the subject's own magical strength to function - it's a piece of ritual magic, if anything (the three times denied request, followed by voluntary death); it's the arrangement of the parts that matters. She might have been a truly powerful witch, or merely a slightly-above average one who was charming and bright. Her not trying to take on Voldemort…well, the not having her wand on her was sheer stupidity, but I can believe that a young woman like her facing a supposed dreaded serial murderer froze in the moment. Logically her attempt to save Harry wouldn't work, but I can just about - just about - buy that she was beyond logic at that point. Although I'm not sure what that says of her vaunted Gryffindor (charge-into-battle type) bravery then. Maybe here her characterization does break down a bit. It's certainly not what we were led to expect her sacrifice to have been like, anyway.
I'm also not clear why on earth it *worked* (beyond authorial fiat). Magic in the Potterverse supposedly is all about will, but she didn't know of any such protection beforehand so she can't have been willing it to happen (and Voldie certainly wasn't). I can't buy that this was the first time in the history of magic that a mother ever begged thrice to be the one killed, not their child, or that Lily's mother love was *that* extraordinary here compared to what mothers in history have done. I don't know why Voldie bothered with the asking and then killing, when he could have stunned her at any point (and if he's a Dark Arts expert of such caliber, did the possibility of ancient magic coming into play NEVER occur to him? wtf)