There are many things that can make a story boring for me. Like others have mentioned, pacing and description are big ones. Both of those reasons are affected by word choice. Sometimes the characters become flat. A lot of what is boring depends on the reader's preferences, or even their mood. My books of choice vary from week to week.
There are many variables that can cause a story to start to become boring in my opinion, heavy plot armor, main characters that are too overpowered, as well as becoming too predictable. Now all of those elements are in almost every story and can be fine in moderation but it’s when they are taken to extremes that I find it troublesome. A character overcoming a dire situation in an epic way is thrilling and makes for good reading, it’s when they face an impossible task but the solution feels forced or they win simply because of them being the main characters.