While being a crazy Call Of Duty 4 fan, I have to say that Indigo Prophecy is probably the best game I've played.
Yet Indigo Prophecy still manages to succeed without having anything required for a gaming masterpiece. All that you need to know is: Indigo prophecy tells a Story, the best story you'll probably see in videogames. The story of Lucas Kane, an ordinary person who got caught in a situation no one would want to be in. The story is practically perfect, it grabs your attention from the very beginning and doesn't let go until the end.
What is especially good about IP's storytelling is that it has a lot of memorable situations. The suspense reaches a new level when you play as a person who has just committed murder in a psychological trance and has to escape the place without raising attention, knowing that a cop is about to enter. Or how about, playing as a detective who gets trapped is an asylum where all the lights have gone off and all the doors holding the psychopaths have opened.
Every downside of indigo Prophecy that you can list is not enough to overshadow its artistic value. Even the fact that it doesn't belong to any of the genres. You can call it an "action button masher" or a "puzzle quest" but it's really not. It is an interactive movie, an Oscar-worthy movie I might add.
I can go on and tell you about the fantastic Quick Time Events (which horribly suck in most games but I feel they're great here) or great resolving scenes but it is not necessary. If you care about underappreciated artistic games, you've already played IP. If you're not, you won't care about this gaming gem.