The original power system in JoJo's was 'Hamon' or the 'Ripple', which utilizes controlled breathing so the user can fill their body and attacks with sunlight energy. Making it very effective against Vampires, Zombies, and Pillar Men.
After part 2, Hamon was just disregarded and then 'Stands' became the new system. I'm not saying that Stands aren't cool, but it just seems like a waste to completely forget Hamon so why did Araki change it?