If it were Burst 1, it would be overpowered. It is not. It allows you to damage any enemies adjacent to the enemy you attack, which is wonderful if enemies march around in columns and lines. In practice it is not as useful as it sounds.
If you are being flanked by two enemies, Greenflame blade does nothing at all. If you are being flanked by 4 enemies and none of them are standing next to each other, the power does nothing. If you have two enemies in front of you, one space apart from each other, it does nothing. Cleave works in all of these situations, and several others. Basically, Greenflame Blade is less frequently useful than Cleave in exchange for being really awesome if your enemies are packed together.