In general good: I like the skull. Thats cool.
In general bad: What's that in the corner?
Details are only relevant to what the piece needs to be in the end. Can your audience tell what it is? How much time to you want to render your subject? Do you need to make it look life like or are you going for looking like a painterly illustration? Its really up to you and the purpose of the finished piece.
If you can tell us your purpose and intent of the piece that will help us critique it a little more directly and help you move forward in how much detail in rendering you think you might need.
Intent would be, "scary", "mysterious", "a story or lore for my intellectual property", "immense scale" "The river styx", "Ancient god still alive", "Nectar of sadness" "I just wanted to paint a huge skull with horns and thought of this".....etc.
You don't want your audience to have questions. I cant tell how big this is unless I know for sure that's a little person in the corner. That person in the corner could have simple shapes and design. Like Mr.Scholes here, his people are small and simple in shape, but clear in design. They dont need a lot of detail. You could use a little work on the atmospheric perspective maybe to make things seem larger and more distant and huge. My friend Alex has some pieces likes this that I like you to look at and see what I mean. When you look at it, really not that much detail is needed. for something huge. But the story is there And Peleng has some cool compositions to show story telling and mystery, but also grand scale.
If you really want to focus more on mood I would really look at Peleng maybe and see how simple his brush strokes are but how much he convey's in his pieces. He's modern. If you want to study a true master and teacher of the golden age of illustration where we modern artists get our rules from start with Howard Pyle and his students. One being N.C.Wyeth. I have learned a lot from N.C Wyeth and his paintings as far as how much detail do I really need to add and what mood to focus on, you cant go wrong there.
