A standard that only covers the protocol, and not the connector oddly. Many vendors used the standard 5 pin microUSB plug for MHL. Samsung caused some controversy by shipping an 11 pin MHL port on the Galaxy S III, which was incompatible with all 5 pin cables. They did also ship an 11 to 5 pin adaptor.

Their main reason for making the change was to allow simultaneous use of the USB and MHL functionality. 5 pin only allows one or the other.

Looks like no matter what ecosystem you participate in, it's a potential mess. 30 pin to 9 pin, or 5 pin to 11.