The reason why I enjoyed and like Kahaani is because the film is primarily a return to clean storytelling. It doesn't deviate from the story too much into unnecessary subplots thankfully avoiding "item numbers" and other such atrocities that have become a common part of Indian films.
Kahaani is the story of a pregnant woman played by Vidya Balan visiting Kolkata on her own in search of her missing husband, who, and this is where the story forms its basis, is also being hunted by the Indian government and police for being a terrorist involved in a bombing. The story, which does have a few loop-holes, works well as a detective piece and having a pregnant woman, gives it an unconventional primary character. Vidya Balan plays the role so convincingly that there were reports around the time of the release of the film that she might actually have been pregnant during the shoot. The reports turned out to be false.
The weakest aspect of the entire film is the opening sequence. What this sequence does is implant doubts in the minds of the audience about the story that follows and that in turn takes away from the deductions happening in the film. Nevertheless, the supporting characters played by Parambrata Chatterjee, Nawazuddin Siddiqui, Ritabrata Mukherjee, and Saswata Chatterjee are so good and so into their respective roles that its hard not to enjoy their characters perform and add their own dimension to the storyline.
The director Sujoy Ghosh makes the most of the vibrant city that is Kolkata making events like the Durga Puja intrinsic to the story and in fact goes to the extent of even making the music by Vihal-Shekar relate to the region.
Kahaani is simplistic storytelling at its best. Staying away from the run-of-the-mill commercial aspects of Indian cinema it manages to attract the thinking audience into enjoying the film all the while demonstrating the wide variety of talent available to Indian cinema in the form of actors, locations, and filmmakers.