Released so far: Mega Evolution (ME01), Phantasmal Flames (ME02), Ascended Heroes (ME2.5), Perfect Order (ME03), Chaos Rising (ME04), and Pitch Black (ME05, July 17)
If any part of that creature's head is higher than the bottom of the bar (that is, if it would hit the bar), the creature is a legal target
This feature helped to make the world much more accessible, and easier to maneuver through
Overlord of the Hauntwoods, Trumpeting Carnosaur, and Kogla and Yidaro all have alternate casting costs that make them quite flexible, able to answer threats more directly than with just another big creature
Bobby Witt Jr
This does carry a benefit in that newer cards tend to be more available and more readily reprinted, but it does run the risk of eroding Commanders charm and personality