This is the question Koku Gamer asks in an editorial. It's fairly short, and doesn't dig deep into the question, and the luster lost seems to be the one from the BIS days, not from anything Obsidian produced.
If I remember correctly, Obsidian's current release, Alpha Protocol, had been in development for a few years. I remember still being married at the time of me sitting on a bed and reading an article on the game that featured screenshots and an incredibly descriptive article that painted the picture of a game that was well on its way to being released. The concept was solid, and I was nearly sold on it. However, the finished product was underwhelming. Though the dialog system was fantastic, the rest of the game was disappointing, to say the least.
Being a fan of combat sports, I wonder if I should draw parallels from Obsidian's issues to an old champion who has lost his chin. You see flashes of brilliance, but at the end of the day both of them ending up flat on their faces, wondering when the better days are going to return. Truthfully, I could be getting ahead of myself, but at the same time it seems like Obsidian has been proverbially shooting itself in the foot since its departure from Black Isle. Each game they've released has had solid storylines and fun characters, but that's not enough to make a great game.