According to JK it's not the shape that's significant. I also remember her saying somewhere else that she chose the lightning bolt because she thought it looked cool. She said something like, "Well I couldn't have him walking around with a doughnut shaped scar!"
I don't remember Rowling ever saying specifically what it meant. (experts, correct me if I'm wrong).
I think everyone has their own meanings for it.
I think the shape represents power.('cause...lightning bolts are powerful).
Or sacrifice. Or it could represent the killing curse itself...it's fast but does a lot of damage. Like lightning.
I've heard someone say it was like a rune that meant "power". JK Rowling is pretty clever when she comes up with stuff...it could be anything.
I belive its a sign of power, Greek mythology Zeus the main god had a lightning bolt
In ancient mythologies from many cultures (Norse, Roman, Greek, Native American, etc.) the lighting bolt would be hurled by male sky gods to punish, water, or fertilize the earth or its creatures. Navajo myths linked it to the Thunderbird, the symbol of salvation and divine gifts. On children's toys, it represents supernatural power. Double bolts, popular with contemporary skinheads, symbolize Nazi power
i think its the sign to denote power-strength and fierceness of a lightning. i mean come on all of us like that, who would be interested in other shapes? it makes harry more cool and makes him harry - different!!!!