ITT: 90% of replies are "hurr you just know or you have no taste"
Discerning bad from good literature is easy. A child can do it. "This book is dumb" is something a ten year old can say.
Why writing is bad (or good for that matter) is however nearly impossible to answer in an abstract way. Bad (and good) writing have so many faces. Let's take two decent writers first because you'll be familiar with them. Hemingway and Kafka. Pretty much anyone on Veeky Forums agrees that their short stories are great.
Hemingway has simple prose devoid of flowery language and complicated constructions. The beauty of his sentences is that they seem universally true and on point. They're short, they get their meaning across.
"Women made such swell friends. Awfully swell. In the first place, you had to be in love with a woman to have a basis of friendship. I had been having Brett for a friend." is one of my favourites. The short half sentence "Awfully swell" perfectly captures the bitterness and pain in having a friendship with a woman you loved. You feel it before you read about the love part. The beauty of the prose lies within it's direct, no-nonsense nature.
Kafka on the other hand has complicated, interlaced sentences which seem to have no end. His vocabulary is sometimes unwieldy. One of my favourite stories, a sudden walk uses a structure of constant repetition. It's essentially describing a certain situation by stringing together loads of if clauses. The writing is impersonal, cold, devoid of emotion.
Both of these are very different yet great. It's the same with bad literature.
The fault often lies in the particular work. With things like "My inner goddess is doing the Hula" it's easy. The imagery is off. Then there's shit like "And from a very tiny, underused part of my brain – probably located at the base of my medulla oblongata near where my subconscious dwells – comes the thought: He's here to see you." It's obvious this is just to show off "Hey I know the word medulla oblongata! I'm smart! You know it too dear reader, yay!" The probably is also horrid. Using an exact scientific term together with probably is just wrong.
Other writing is bad because things are straight up told instead of shown. Think again of Hemingway, how he implies all the pain in the example above. Here comes Dan Brown. "Renowned curator Jacques Saunière staggered through the vaulted archway of the museum's Grand Gallery." Literally the first word in the fucking novel makes the entire sentence shit. This is material for a newspaper if some famous person died, not a novel.
Then we have constant overuse of adverbs. "he said jokingly" - He jested. "She said threateningly" She threatened. "He walked silently" - He sneaked. Too many of these make prose seem weak, passive.
There's also lenght of sentences, cacophonic combinations of words and loads of other things but you get the point. You need to work with a concrete piece of literature to tell why exactly it's bad.