Sonic collects rings. Apparently that's a big difference.
Seriously who doesn't know that Mario collects coins? SNAAAAKE have you ever played a Mario game?
Anyway...
A big enough difference not to get sued apparently. Face it, Sonic is a poor man's Mario. They were trying to hard with him... the trailer for the Robocop reboot comes to mind:
"Hey that's not edgy enough, let's paint him black and have red lights and make him move faster and stuff".
It's like the marketers never noticed that people liked the old Robocop (aka Mario) just fine the way he was.
Of course some Sonic games are playable and actually fun... I doubt I'll be able to say the same thing for the Robo-reboot.
But that aside it's kind of strange that since SMB came out nearly all platformers, even the Nintendo-owned DKC have a "collect 100 widgets to get an extra life" mechanic. You would think if you were going to rip-off SMB, you would copy the spot-on jumping mechanic.
Neko-Mario (aka Cat Mario) is pretty awesome actually. He can climb walls, do a Kung-Lao-style diving attack, charge at the enemy, and something Mario has never been able to do before, he can directly attack the enemy via clawing him. Seriously after all these years you'd think Mario would have learned how to ball up his fist and throw a punch, but hey, clawing is better.