Disappointing

What did you think it was lacking?

I know what you mean, I too watched it with a raised level of expectation and was disappointed on first viewing. I went back to it at a later date and enjoyed more on a second viewing possibly because of lower expectations. It has gone on to become a movie I quite like.
Do watch it again.

You might want to watch it again with some context in mind. The director Kubrick filmed this in London with a small budget. Out of that measley budget he was able to created high art. Yes, today a lot of it looks dated or silly but man, look at what he did with what he had. Very shortly after this films release we started getting movies like the Godfather, Exorcist but for it's time Kubrick went up against French Connection, Straw Dogs type movies and in this respect the movie holds up exceptionally well.

Honestly I think it was a very good movie, but not the type you would sit and watch over again, it was good in the sense that it captured the mind of a psychopath so well. Kubrick’s acting was amazing, so much so that it made me feel disgusted in him, which is exactly what I like from an actor because if you dislike the character it shows how well of a job they did playing it. It shows a social concept of a corrupt government and I personally would recommend it to a crime/ horror lover who loves a conspiracy.