Thank you all for your responses and your input, and thanks for welcoming me to the forum. So the mercurial greatsword is from a story that is basically more of a sci-fi/fantasy genre than a straight up fantasy genre? I always did get the impression of 'Final Fantasy' when I considered the merc greatsword.
Anyway, I suppose some of us can agree to disagree, but I feel the mercurial greatsword is the most broken weapon I've ever seen. Drawing comparisons to a scythe...hmmm... ok, I see your point somewhat, but I think I'd rather see someone trying to abuse a scythe (as silly as it may look) than a merc greatsword. It isn't so much JUST the mercurial greatsword as it is the character wielding it. Ok, let me give you an example of what the character wielding one of these in the campaign I am running. He's a half-orc fighter/weapon master; with feats that are geared towards getting the most he possibly can out the particular weapon he uses (improved crit, power critical, specialization). I don't think some of you are taking into account such things when seriously weighing the differences between a standard greatsword wielder and a mercurial greatsword wielder.
Yes, the threat range is lower on a merc greatsword, but a merc greatsword crit has more potential to kill a foe in one blow than a standard greatsword does. I don't even think a greataxe is as deadly.
I thank you for some of your suggestions, but I do not know as though I can bring myself to sunder a character's favorite weapon. That isn't normally what I like to do and I don't think I've ever used the Sunder ability on a player yet. Sundering is not as easy as it sounds to begin with anyway. As I stated previously, this particular merc greatsword is magical; a mercurial greatsword of the planes. It just so happens that the creatures that they are fighting most of the time are outsiders, and none of them wield +4 or better weapons.