Spotting a bad movie - on a personal level - is easy!
The trick is to be able to work out exactly why it is bad!
That, again, can be a tricky thing based on personal interests/prejudices.
I remember saying to someone that I really like "Waterworld" , and being told that it was just another boring post-apocalyptical movie.
Well, my response that was that every movie is basically the repeat of a previous theme/storyline ( be it gangster, love-story,western, sc-fi) but that "Waterworld" was the first post-apoc movie set on water! Breathtakingly so, I felt!