Both feature a wanderer in a future wasteland who fights to protect the weak by making his opponents explode. Both also have very creative (if limited by budget) fight scenes, and as a bonus, the villains Braiking Boss and Raoh share a very intimidating voice actor. (Actually read Hokuto's manga, it's way better, but we can't rec manga to anime really. Casshern has no manga.)
Casshan & Devilman are superheroes who sacrifice their humanity to wage war against evil. These heroes fight out of love for their parents but are feared by humankind. The only people who accept them are their girlfriends; Luna & Miki.
Both are about humans who, through different means, become super-powered cyborgs. Both series have the characters fighting crime and evil. This was a big theme in the 60s with the advent of technology, especially of the robotic type.