Dark Knight 5.0 animations

[TouchandFeel]

snip

You are mostly correct, but not entirely.

Greatsword is indeed a more modern term that was sort of just used a general descriptor for any very large sword, with most European variants usually ranging about 150cm to a bit over 170cm, basically being about as long as the user was tall. For the most part, until the latter parts of the Middle Ages, formal sword classification was virtually nonexistent and most swords were often just called "sword" in whatever originating language or dialect, or some made up descriptor.
Greatswords, as a classification, encompass an enormous variety of swords which include everything from large edgeless swords meant solely for thrusting, to swords like the Falx or most any two-handed sword from Japan or the Middle East which were primarily cutting/slashing weapons.

If we were to hone in on solely European double-edge straight-bladed swords, which is what most people think of when they think of "greatsword", they were actually a very versatile weapon with some fairly complex fighting forms developed for their use. This included everything from sweeping cuts and slashes, defensive stances with the sword extended out to poke at the opponent to keep them at distance, "half-swording"(gripping up on the lower part of the blade to bring the blade in for better control in closer quarters), there was even a technique where you would grip the sword backwards by the blade and bludgeon your opponent with the pommel (better be wearing gloves for this one). "Greatswords" were not really used predominantly in one single way, they were used in a lot of different ways that all depended on the situation that the user was in.

As for ease of use, even well made ones were cumbersome to use due to length and weight. While proper weight balancing would could make them easier to use, they still required proper training to use effectively. It was much easier to pick up a basic one-handed arming sword and use that with little to no training.

In regards to slashing versus piercing, that was a general evolution that you saw in virtually all European swords during the Middle Ages due to the continuous advance of metallurgy and armour. As more advanced armour, such as plate, became more prevalent, slashing with a sword became less effective since the armour would tend to deflect the blow. Swords then began to be designed with more of a tapered triangular profile, coming to more defined point to be used for thrusting between the gaps in the armour.
You didn't really see slashing swords in prominent use in Europe again until much later where the adoption of guns on the battlefield made armour obsolete, at which point sabres and similar cavalry swords saw frequent use up until swords pretty much disappeared from the battlefield entirely.

You are correct in that most historic versions of a European two-handed sword would be much smaller, lighter and way faster than what is often depicted in games, shows or movies where they seem to be swung more like giant axes, but historical accuracy tends to play second fiddle to simply "looking cool". After all it's a game in which we fly around on giant chicken-ostriches, magic is a normality and we travel by teleportation via floating crystals; it's best not to worry to much about how it measures up to real world accuracy. The swords the Dark Knight wields in the game is even described in the beginning job quests for it as being ridiculously large to the point of seeming like it couldn't be wielded, so it wouldn't be illogical to expect it to not be used in the same way a historical two-handed sword would be used.