Actually, according to the live commentary description, the Charge Buster is a smash attack. The actual moves are:
Jab - Buster (Can shoot 1 to 3 shot bursts. Disappears after traveling about 4x the length of MM's body. Can move while shooting.)
Ftilt - Never shown
Utilt - Mega Upper
Dtilt - Slide Kick
Dash Attack - Top Spin (Luigi turns in disappointment/jealousy that one of his moves got copy pasta'd onto someone else)
Fsmash - Mega Buster (Projectile disappears after traveling 4x the length of MMM's body)
Dsmash - Flame Blast (Lasts longer if charged, multi-hit, low knockback)
Usmash - Spark Shock (Sakurai was trying really hard to land this attack but never could)
Final Smash - never shown
Nair - Buster (Only shown to shoot 3 shot bursts)
Fair - Flame Sword (Single-hit)
Bair - Slash Claw (Multi-hit)
Dair - Hard Knuckle (Megaman shoots a projectile downwards that meteors)
Uair - Tornado Blow (Megaman spins and fires a tornado upwards)
B - Metal Blade (You can throw this in 8 directions, if it hits the floor, anyone can pick it up)
side-B - Crash Bomber (Like brawl's gooey bomb)
up-B - Rush Coil (Copy of Sonic's spring, no special fall)
down-B - Leaf Shield (Not sure if attacking causes you to throw it or if it stays until down-B is inputted again. Leaves are multi-hit but not guaranteed to be multi-hit. Lol brawl style attacks not linking into self properly.)
Ledge attack is a typical kick, and his get up attack is extremely similar to Mario's sweep.
Source:
http://www.youtube.com/watch?v=rqxDPJmO6f8 and Developer Direct.