C is used for specials. You still have the "combo melee" on mouse 2.
Basically once you have enough bonus points to use a special you press C and an aura appears round your character (only you can see this aura, the other players can not see it). The aura will stay for X amount of seconds based on the tier of the special you selected (pressing C once, twice or 3 times).
If you hit someone during that time the special attack will trigger instead of the regular attack.
Regular attacks are still mouse 1 and/or mouse 2 being held down and flying in to someone.
Mouse 1 = simple melee (1 punch and the other guy goes flying)
Mouse 2 = combo melee (a series of 3 to 5 attacks depending on charge time before the opponent goes flying)
Mouse 1+2 = throw
Simple melee works like 1.2 with a longer knockback distance, combo melee is something in between simple melee and a throw and a throw is just a throw.
In terms of DPS potential simple melee is the highest.
Second is combo melee as a minimum charge does less damage and holding on to a full charge has a penalty, but you can aim where the opponent will be knocked to or you can use the direction keys to knock him in that direction.
And last is throw, that doesnt do any damage, but you have controll of direction and distance of the opponents knock back based on charge time.
A minimum charge throw will position the opponent right in front of you perfectly set up for a hit with a different attack, while a fully charged throw will send him flying further than simple melee.
Ofcourse starting all your attacks with a speciffic melee means that people can use the RPS setup of the system to counter you.