Actually, I find the syntax highlighting somewhat irritating.
When I select some text I don't really want the surrounding code tags highlighted too - they are colour coded, for me that's enough.
There's also a lag before before it does the highlighting, I find that distracting, I'm half way through typing something, suddenly the tags are highlighted, then I discover oops I've hit ctrl+x or something.
When I use control+right/left arrow repeatedly, to skip through words, the code highlighting gets totally out of synch with what I'm doing. If I don't step over a code boundary there should be no need to refresh the highlighting.
My suggested solution would be a preference to toggle it off.
Another irritant is that the context menu for selected text often covers the selected text - Sigil's not perfect in this, but definitely better - so is firefox I just noticed. In some situations Word's just as bad - but Esc then Menu key usually fixes it.
If I press menu key after pressing F8 for next spelling mistake I don't get a menu that includes suggested words, to get them I have to mouse right button click the highlighted word. In Sigil a tap on the menu key after F4 delivers a context menu that includes the suggested words.
I can't see the point of spell checking Comments in the content.opf - the definitive copy of Comments is in the database, if I correct them in the epub the changes don't flow through to the database - nor would I expect them too. A check box preference for "Spellcheck content.opf file" could fix this.
Whenever I consider switching back to the calibre editor, which is at least once a month, these are the sort of issues that deter me. For me at least, it's the little things make a big difference to usability.
I don't need most of the 14 features you've mentioned - I appreciate my 'don't needs' will be different to the next person's 'don't needs'. Some calibre features are handy to have, such as Live CSS and the Links and Words Reports - but I can use/get them when I need them.
And so it goes...
BR