Beware: Spoilers ahead
As a standalone film, The Thursday Murder Club is about a 3.5/5 star film. As an adaptation of a truly wonderful book, it is a 2/5. I think it largely fell flat because Netflix chose to adapt the book into a two hour film instead of a longer miniseries.
I went into watching this with high hopes. From the beginning, the feel was right. The casting (with the exception of Chris) felt spot-on. More on Chris later.
The biggest issue was that the writers/director decided to both condense and slightly change the plot in order to fit a two hour film. The book is partially so good because it has depth. It has subplots. It fleshes out more minor characters and is able to pursue false leads and red herrings. The film, on the other hand, is more straight to the point getting to who committed the murders. Most of the false leads are not really touched on, and the ones that are included are not explained particularly well. Almost all subplots are excluded. And *SPOILER* Bogdan's exposure and arrest as Curran's killer in the film (in contrast to only Stephen learning of it in the book with someone else, conveniently long-dead, taking the fall) is not keeping with Bogdan being a recurring character in future books. Plus the motivation of the murder is completely done away with.
I also take offense to how the film handled Chris Hudson. He is slightly slower than Donna to come around to the Thursday Murder Club in the book, but he does cooperate and is truly a team with Donna. The film completely does him a disservice. He is just a comedic bumbling buffoon, with Donna and the Thursday Murder Club running intellectual circles around him and quite literally getting to everything slightly ahead of him.
As previously stated, this is not a bad film if taken without comparison to its book material. It is charming and enjoyable, but doesn't have quite as much depth as other better murder mystery films. Yet the film is a major disappointment when compared to the book it's based on, and has shot itself in the foot for potential sequels.