I watch films and discuss very seriously. Just kidding folks :)
The reason this worked so well for me was Elliot Gould as Marlowe. The film is often funny with most of the humour coming from Marlowe's interactions with characters in opposition to him, with him saying smart witty comments, but there's a tiredness to it even if he's being effortlessly cool, he's trying his best to find out what's happened but it's hard for him. Some scenes relating to the plot were uninteresting at points but the film was always engaging with Gould's incredible central performance and character.
Was probably an inspiration for 'The Big Lebowski'. Anyways, great film.