In this article Knuth talks about the bottom 10 problems rather than top 10. The problems which are not solved but they are most ready to be solved of all.
What are they?
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Well... this is somewhat subjective but...
I think Knuth is hinting that most mathematicians (and I'm guessing most computer scientists) work on problems that are not particularly glamorous and not immediately important, nor do they have particularly elegant proofs.
The use of the phrase "bottom ten" appears to be a metaphor, referring to problems that could be solved (using well-established techniques), but haven't yet been solved. There's no actual list.
The "stones that make up the wall" refers to that important mathematical discoveries are typically not made by one person working hard on a problem, but by a host of contributors over many years, each making incremental progress on the problem.
PS. I can understand that Knuth's response might be surprising to people without research experience. Why would he want to work on the least interesting problems?