If a player went and broke another players legs with clear purpose they would be banned from football for life and probably placed in jail. There's a difference between going in for a wild, reckless tackle and actually trying to hurt somebody on purpose, with the only objective in the players mind being to cause harm.
Suarez has bitten somebody three times. What else does that mean he is capable of on the pitch? If I was a player I'd honestly be wary of going out on the same field as him, if a guy has the capability to do something as absolutely crazy as biting somebody and is a serial offender of this, he really shouldn't be allowed to continue playing. It's a seriously disgusting act. People stamp out of aggression, which whilst abhorrent is understandable when the heat rises. Biting shows a certain lack of rational thought, it's not a rational response to being provoked, which he actually wasn't. It's quite frightening to be bitten, imagine you were just busy playing football and somebody decided they'd have a nibble on your shoulder. He's already shown he's capable of other acts of violence, so a lengthy ban could be a deterrent to him not just to stop going around biting other human beings, but to stop other ways he'd like to hurt them.
Spitting at another player holds a large ban as well. It, like biting, is considered a disgusting, hugely disrespectful and cowardly thing to do.