There's more than a mother's love. That love is what drove her to sacrifice herself for her son, and that is something Voldemort couldn't break through. That sacrifice. There are more sagas in which sacrifice grants protective power.
In Harry Potter it was mentioned so much as it was used later on, but many people missed it.
You remember Dumbledore telling Snape that Harry had to die, and that it was essential that Voldemort would be the killer. What he didn't mention to Snape is that Harry had to offer himself up to Voldemort of his own free will, and Harry did that. Harry did so, hoping that way his fellow students and his teachers wouldn't have to die. Harry sacrificed himself like his mother before him. Now Harry didn't die, but Harry didn't know he'd have a chance to survive this sacrifice due to some other mistakes Voldemort made. And Harry brought the same protection over his fellow students and teachers and other people in Hogwarts the same way as Harry got protected by his mother's sacrifice. Since both Harry and Lilly only offered themselves up to Voldemort alone, the protection was also against Voldemort alone.
Nice detail is that Voldemort failed to make one more kill after hitting Harry with his killing curse and the Limbo scene that followed. All kills after that were done by the Death Eaters. Harry even confronts Voldemort with the facts I just mentioned during the final confrontation, however Harry's final words to Voldemort were all omitted in the movies. (Except for him being the true master of the Elder Wand, however in the books he explained this to Voldemort (who didn't want to listen) and in the movies he told Ron and Hermione after Voldemort was already defeated).
The power of sacrifice has always played an important role in fantasy stories. Many sagas bind magical protection to sacrifices. I believe there are even some stories in ancient mythology where sacrifice has this kind of power.
Lily's love was only the reason why she'd sacrifice herself for her son. But it was not fully love itself that brought the protection. It was the sacrifice she made because of it. Of course Harry had, despite being raised in hatred, kept his mind pure and hated hatred itself, and could still love (the power the Dark Lord knows not), and thus he was willing to face his fate, knowing he should be dead afterward.
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