Figured I would start a dicussion on this as its a pretty good topic. Not to mention write something little more on target compared to my other flippant post
I think a mech should be defined as a single occupant Environmental Combat Suit regardless of its shape or form of mobility. Thinking various 'suits' could be tailored specifically to roles, from bipedal and quadripedal to tanks and wheeled vehicles. Weight requirements would fit significantly into the form somewhat similar to how Chromehounds and Armored Core.