That being said, Monster Musume is *not* a generic anime however. Rather it has a quite fresh take on the genre with a very unorthodox character cast which indirectly makes the ecchi elements very unique. It's very creative for a fanservice anime, and thus manages to stand out from the crowd quite a lot. As a result, Monster Musume is one of the more noteworthy series in the ecchi harem genre.
First of all though, I'd like to make a general statement here. Given how many complaints you see about how many bad ecchi harems there are nowadays, have you ever stopped to ask yourself what exactly it is that makes a fanservice-oriented ecchi harem *good* then? Of course this is an entirely subjective question, but personally I can think of a number of things as signs of one. I'm going to go through them one by one and then compare Monster Musume's performance to those standards, and in that way analyze the contents of the anime.
1. A good ecchi harem should not have any sort of cheap excuse story. Such a plotline is only going to suck anyway so don't even try.
Monster Musume's premise is rather simple. Due to a new law, various kinds of monster girls have been allowed to move to human countries as long as they are taken care of by a human host. This is in order to see if different species are capable of living alongside one another in the future or not. And thus the story is about an ordinary guy who gets tasked with taking care of a bunch of sexy monster girls of various species by cohabitating with them. Unfortunately for him, it's still highly illegal for humans and monster girls to mate with each other, and thus he has to muster up all the mental strength he has in order to resist the girls' rather forceful attempts at seducing him.
And that's about all there is to it. No complicated overarching story, no "dark past", no stupid out-of-place action scenes or anything along those lines. Just plain and simple lewdness in a comedic setting. That's all it is, and that's all it needs to be. If you watch an anime for the sake of ecchi... well then you don't need much more than ecchi as long as it is executed properly.
2. A good ecchi harem requires some sort of originality. There needs to be something to make the anime stand out, otherwise it will only come across as generic and repetitive.
Monster Musume's heroines are very unique, for obvious reasons. We have a lamia, a harpy, a centaur, and tons of other species making up the female cast of this show. While this is unusual in and of itself, it also provides an almost infinite possibility for creative fanservice. Since different characters have completely different body parts, physical abilities, reproductive functions, and views on common sense, we end up with a show which has sexual content that I can guarantee that you've never seen before in an ecchi anime.
3. A good ecchi harem should have a respectable male lead. We don't want an annoying pervert, a spineless coward or a rapist feeling up the girls exactly.
The protagonist of Monster Musume is called Kurusu Kimihito, and while there's nothing remarkable about him, I still consider him to be a pretty good harem protagonist. To start with, he is reasonably mature, and manages to keep up with all the girls advances fairly well... as long as he's physically capable of resisting them that is. He's also got a rather admirable personality and simply feels like a good guy all-round.
4. A good ecchi harem benefits greatly from having well-written and likeable heroines. You're going to be way more interested in seeing a girl you really like strip for you than a girl you couldn't care less about do the same.
Here I think a distinction needs to be made: I already clarified that the heroines of this anime are very creative and unorthodox. However that is solely because of their physical appearances and the high potential for fanservice which comes along with that fact. But that is also the only thing which makes them special. As far as personality, character development and simple likability goes... they really don't have that much despite everything. Of course you might argue that an ecchi anime doesn't need those elements, but I'd like to disagree. Would you feel exactly the same if your girlfriend did lewd things to you as you would if your girlfriend's identical twin sister did the same? I'm going to say no. I think that if you like someone on an emotional level then whatever erotic things that person does would come off as way sexier. And for the same reason, despite their interesting physique, the girls of Monster Musume fail to establish any deeper connection with the viewers, and thus are unable to live up to their full potential in terms of eroticism.
5. A good ecchi harem must absolutely have good art and character designs: When your goal is to make the heroines appear sexy, they should obviously look good as well.
Monster Musume's art looks fantastic however, so there is no need to worry about that. While it looked good in the manga, I think it looks even better in the anime due to the crisp and vibrant colors. Everything looks incredibly clean and detailed, and consequently all the girls also look amazing.
6. A good ecchi harem should have intense enough ecchi elements to actually be hot, at least if it's a fanservice-oriented one such as this. Just some minor teasing every here and there just doesn't cut it, it's too softcore.
This category is rather hard to properly analyze as of yet in Monster Musume's case, because unsurprisingly the TV-airing of this anime is censored. As usual, you'll have to wait for the BDs to see this anime in all its naked glory. Though I have to say that the censorship in Monster Musume is very tastefully done compared to what I'm used to seeing. There are almost no light rays, steam/smoke or weird graphics blocking up the screen in this anime. Instead they resort to having nipples just barely blocked from the camera by having objects in the way, or zooming in the camera in order to leave the NSFW elements just barely outside the edge of the screen. These actions can then simply be reverted on the BDs. As a result, the censorship doesn't really distract you when watching the show, which is something I appreciate very much.
Censorship aside, Monster Musume has a lot of quite high quality lewdness. Although I doubt I'd use it as fap material exactly, I've definitely gotten some hard-ons while watching this show, and I think that speaks for itself pretty well.
7. A good ecchi harem should also have sexual content with appropriate timing. Don't throw in random panty shots when it doesn't make sense or the mood just isn't there; the story should support the fanservice, not the other way around.
Given that the girls' relentless sexual assaults on the protagonist is one of the core elements of the entire story, the ecchi segments are most certainly justified and believable for what they are. Of course some scenes might be a bit more far-fetched than others, but they never feel poorly timed or distracting from any other aspect of the show.
So if you put all that together, I think Monster Musume passes every single category except for #4 (#6 can't really be judged properly yet though). And that is the only reason I'm not giving this a higher overall rating than I am. I think what Monster Musume really needed to have in it is a girl like let's say Momo from To LOVE-Ru Darkness whom in addition to being sexy also has an enormous impact on the entire story as a whole and makes every moment more interesting. But it doesn't, and as a result Monster Musume eventually starts feeling somewhat repetitive. It basically runs on a "girl of the week" type formula where new monster girls keep being introduced to the cast and their unique ecchi potential is investigated. But once that is done... well then the characters don't really feel very interesting anymore because again, they don't really have anything going for them beyond their physical characteristics, and thus many of them are just tossed aside afterwards. And because of that I feel like the show lacks longevity.
However that being said, this is only a 12-episode anime, and it still certainly manages to keep you interested for that long at least. If it had been like 50 episodes long then it would probably have been a lot less impressive as a whole, but for what we've gotten thus far I don't have very much to complain about overall. Monster Musume is one of sadly few ecchi harem series which actually tries to think outside the box and deliver a fresh viewpoint on the genre as a whole. It's both fun and exciting to watch, and almost never fails to make you smile when watching it. And really... I don't think you can ask for a whole lot more than that from an ecchi harem anime.
