It's hard to be short and concise with a review of Mythos: Book One, but here goes:
Humor as a selling point: The game is hilarious. The dialog is well-written, either by a native English speaker, or involving someone with an excellent grasp of English (worth mentioning as many games in these genres are sub-par from a language perspective).
Presentation as a selling point: From the transitions, to the introduction of characters, this game has so many things going for it. The renders are solid, the animations do the job and are smooth. There is humor in the presentation itself, beyond the dialog. The addition of the lore book is great for remembering, and very welcome in that it doesn't restate, but rather rewrites with alternative descriptions, what you may have discovered about a character.
Narration and Storytelling as a selling point: A good mystery is presented early on, and the developers do a great job of dropping enticing cookie crumbs as you progress at an inviting pace, keeping me interested in the story itself, certainly in addition to, and either in lieu of lewd scenes, or simply bridging them. As other reviewers have stated, this is a good story that has lewd scenes. Not every AVN pulls that off, but Mythos has so far, for me.
Perhaps the best single thing I can say about Mythos: Book One, is that I have chuckled, or snorted, out loud more than once from legitimate, intended humor. That alone is enough to keep me clicking.