I appreciate the clarity of your message... curiously enough all the things that you mention I see them as anti-features of nushell (except the first one)
1. Posix shell is clunky has its limitations but there's nothing really flawed about it. It is nice to have non-posix shells, though; in that I agree with you.
2. I hate hate hate the very concept of dataframes. They seem like a useless over-engineering. Plain tabular data is a perfect, and the columns of a table are always named 1, 2, 3. The whole point of abstraction is that you do not care about the meaning of the columns of your table, and the programs (like the lovely awk) that deal with these columns do not care either. There's nothing more "orthogonal" than that: filters that do not understand the data but can filter it anyway.
3. syntax highlighting is a very personal issue; in my case I prefer my terminal not to look like a christmas tree.
4. xargs is one of my favorite tools (alongside gnu make and gnu parallel) and there's nothing "unserious" about it. The happiest moments in my programming life are when I get to use these tools. Not using xargs sounds like a nightmare!
5. Rust is definitely an anti-feature to me: it's like a language that was built by picking the worst features of many already ugly languages. I would not really care in what language the shell is written, but I'd rather have it written in saner, "unsafe" languages.