Here is the stack. At the bottom level: erudition.
Erudition means knowing things at a deep level, or at least knowing something at some level.
One step up from that is critical thinking. Being able to perform basic operations about what one knows, comparing and contrasting, summarizing, analyzing texts, drawing conclusions from evidence, applying theories to concrete instances, etc...
[Then comes creativity, the ability to have truly interesting ideas emerge from these thinking operations.]
The next thing, I would say, is nuts and bolts. This is being able to construct paragraphs and essays that put together the first three levels in a convincing way.
[Finally, there is kind of stylistic elegance, that might emerge from someone who does this in a satisfying way. ]
Someone who was at a B level or above at all of these things would be a great literary critic. A true failure at any level would be a serious flaw. I put creativity in brackets because, while it is the most important thing at some level, it is also the rarest. You could do everything else, without much creativity, and still be satisfactory. Yet for me, there wouldn't be much point to doing all this and at the end not having put forward really great ideas.
You could also get away with inelegant writing and do fine in the field, so I'm also putting that in brackets.
So at some level the most basic "stack" would be
nuts and bolts of essay writing / basic critical operations
knowledge or erudition
[Maybe nuts and bolts and critical thinking is the same thing. In other words, knowing how to compare and contrast, etc... is the same thing as knowing how to write an essay that does that. I'm not sure. For example, perceiving things about a poem is not the same thing as writing an essay about those perceptions. So maybe analyzing the poem is part of the knowledge about the poem?]
Of course, I think I'm good at all of this! I know people more erudite than I am. There are critics who write more elegantly. Many, many people have mastered the nuts and bolts of essay construction about as well as I have. Probably thousands of people are better than I am at this. The thing about the stack, though, is that having all the layers really solid is what is rare. Mere competence, then, becomes excellence.
The reason it is a stack is that one skill depends on another. You cannot do thinking operations like comparing unless you have something you know, that you can talk about. You can't write an essay unless you know how to compare two things, or perform other critical thinking skill, like argue why or why not a concrete example of something confirms or disconfirms a more abstract theory. You cannot write elegant prose unless you are first doing some writing.