Mk2 is certainly a whole lot different from the original, in large part due to its M rating, completely redone battle system, 3D models to replace animated 2D ones during story sequences, more content, more endings, and generally, a lot more stuff.
Mk2 can be summed up very quickly to compare to its predecessor, as both are founded on the same premise: A metaphor for the console war between Microsoft, Sony, and Nintendo, along with the ever-increasing threat of piracy from Arfoire (get it? R4?). The game was designed to be totally off-the-wall. The original was very entertaining in that the writing was superb, and very funny, constantly jabbing at parts of the video game industry combined with a multitude of breaks with the fourth wall, with the characters essentially knowing they're part of an RPG. You played not really for the fights or even the main story as much as you wanted to see whatever they could come up with next to parody, as they touched on virtually everything in the game industry. The whole thing was about humor, and to some extent, fanservice and flash. It was T-rated fanservice, sure, but it was fun, and the battle system, while very simple, was well-crafted and left room for creativity.
Mk2 is kind of like the original to the power of three. This game takes the original premise and runs the whole field with it. The opening sequence alone shows you why this game has an M rating, with less-than-subtle erotic animation of the scantily-clad CPUs in erotic bondage poses, complete with Gainax boob-bouncing and some extreme sexual innuendo.
Oh, yeah, about that sexual innuendo. I'll say it right out: A *lot* of it has to do with lolis, and there is no ambiguity about that whatsoever. You'll even find quests in which your job is to find ingredients for a team to make potions that will create, and this is a quote from the game itself, "lolis with big knockers". While the original kept its T rating intact, Mk2 throws all pretense out, complete with borderline-illegal loli fanservice. They don't even try to cover up that it's loli by saying they're older than they really look, which is what developers often do. One of the main villains is obsessed with Lowee's (the Wii's area) two CPU Candidates, Ram and Rom, who are two very small, young girls, one being exceptionally shy and timid, the other a little less so.
And when I say this particular villain is obsessed with these two lolis, I mean he flat-out says he wants to molest, lick, play with sexually, and possibly rape them. There's even a reference to his being a "cunning linguist", and he states clearly that he doesn't accept anyone who is past puberty. Further, some of this he even does, clearly getting off on it while the two girls are sickened. He raves nonstop of his completely unrestrained pedophilia, which isn't the slightest bit censored (hello, M rating!).
Oh, and here's icing on the cake about Ram and Rom: One of the endings involves an event that I literally didn't think they would go through with. I thought there had to be some way to get around what they were insinuating what was going to happen to both of them. I thought that to show that kind of thing was simply illegal. Apparently not, since the entire scene plays out without holding back in the slightest. It *will* make you at least a little uneasy as it plays out, especially because this particular scene does not involve any humor whatsoever, but tragedy and horror--it's meant to stir your emotions in ways almost all media doesn't dare. This particular tragedy is something I thought could not be shown in video games, or practically any type of media. I was floored that they went through with the scene, and further floored *when things just keep getting worse when you think they can't possibly get any worse*.
One major drawback that immediately turned me off was that the amount of voice acting has diminished very significantly. While they thankfully brought back all the VAs from the previous game, who all did a spectacular job in their roles, it seems random as to what scenes get voiced or not. Many scenes, even critical story scenes, do not have voice-overs, which is downright puzzling, and it definitely detracts from the scenes. The original included nearly every story sequence having voice acting, but here, even story sequences often don't get any. It's pretty lame. Still, the quality's there.
In terms of humor, you can tell it's not quite as inspired as the first. It has its moments, for sure, and it gets a lot of those moments by pushing the boundaries. Sometimes it even goes for very dark humor. There's about five times as much sexual humor, but it's done in a way in which the characters on-screen are typically unhappy to be shown on your television screen that way, with several sequences where the fourth wall is broken--not only is it broken, but another character announces that things must be getting bad, because the fourth wall was just broken, as she announces. The original generally had better writing, in no small part because Neptune was designed to be humorous, but our CPU Candidate here, the main character Nepgear, has no sense of humor at all, and most jokes are at her expense. But it still doesn't work the way Neptune really drove the story for us in the original, and Nepgear isn't as entertaining as Neptune was.
This is, however, mostly a typical RPG, and you may need to grind as well as farm to complete quests. There are two types of quests outside of the story, and two only: Collect a certain number of items, or defeat a particular enemy or enemies. Drop rates do apply, so you may slay five of the enemy who have a chance of dropping the item you need, and yet still, sometimes none will drop at all. Most mechanics in the game are typical RPG-fare, although it's still done well. It's great that enemies are visible on the map, so you can dodge as many as you want, should you choose to, which is particularly helpful if all you're doing is going around gathering herbs or something, or looking for a very specific enemy. The combat system, while more entertaining than the first, isn't quite right in that the transformations are now totally worthless, their cost far outweighing the benefits involved. Costing a whopping 100 SP to transform at all with a serious drain on the gauge every turn you're transformed, you're almost always better off spending it on a special move in your normal form. The only time you can really use your transformations is if you've saved up SP in a single dungeon before a boss fight, but even then, at some point in the game you will acquire a skill that involves all four party members doing a team move that will almost always be a one-hit kill or very close to one, even for late-game bosses. If you combine this skill with the True Goddess Aegis item that removes the damage cap to above 9999, the damage is often around 30,000 per hit...for 15 hits every time you use this skill, which will be instantaneous death to any enemy you come across, bar none. It's a sure-fire way to win any fight if you prepare ahead of time with 100 SP to spare for all of your characters at the beginning of every fight.
I think I've touched on the most important parts: Fanservice in this game has gone to level 11, warranting very well its M rating, particularly with the clearly pedophilic recurring side-antagonist openly sexually harassing and sometimes abusing the two youngest CPU Candidates, Ram and Rom, who look to be maybe nine or ten years old each. The game pushes its concept even further than the first, although not always with success, as it's just not quite as funny.
One final note I'd like to make is that the developers has included a somewhat difficult-to-obtain and surprisingly lengthy bonus chapter immediately before the final boss. This chapter isn't exactly difficult to unlock since the conditions are fairly straightforward, and you can even wait until almost the very end of the game to meet the requirements, but it's definitely time-consuming, and if you're not going for it, there's no way you'll accidentally get it. However, know that if you choose to unlock this new chapter, get ready for a complete and total 180 in the mood. What was once a game based in humor is now based in raw, unrestrained tragedy, one that does not follow the rules of this kind of thing. The game will suddenly become horrifically dark, and words can't describe the sheer shock value of some of the things that happen during this replacement ending chapter. To describe it without spoilers is impossible, since the entire story suddenly takes a new turn, and any mention of any event, even hinting at it, would spoil it. Let's just say you're in for a hell of a depressing ending beyond all measure should you choose this route. We're talking witnessing children suffering violently (complete with English voice-overs that somehow make it worse, considering the manga theme), suicide, forced murder, mass murder, one particular character smiling and asking you to kill her (considering the importance of this character and how familiar you are with her and her story by now, this scene is just shocking), cries for help that only end in slow, drawn-out tragedy, and plenty more you simply will not expect in a million years in a game like this. The scene when a particular character takes you outside to make pleasant small talk while you're already in a traumatized state from the events unfolding, and then casually smiles and asks you to kill her so she doesn't have to do it herself, along with everything that follows, will blow your mind to hell and back.
So, there you go. It's worth it, for sure, if the premise interests you, no question. It tops the first in almost all aspects, and is an excellent successor.