Tough questions!
What is the biggest difference for you between Excel and Coda?
- Coda’s modern look and user-friendliness.
- The need to use formulas at the row/column level only in Coda, instead of cells; when I discovered this kind of spreadsheet formulas (e.g., typing “A:A” once instead of “A1” with a down-copy), for instance in Gsheet conditional formatting, I’ve been hooked instantly and wished I could use it more widely. Same with named formulas/ranges, although I hardly ever used these before Coda due to lack of “Return on Investment”.
What was the hardest thing for you to learn when getting started?
The same as now: wrapping my head around the way Coda formulas work. I’ve been using Excel, LibreOffice, Gsheets and dozens of programming languages for nearly two decades, including low-level and high-level languages, all sorts of editors, and all-over orientations like database/machine/human/object/math/function/declaration/etc.; yet many of the formulas I try in Coda don’t work: sometimes I manage after a few trials, and sometimes I give up due to lack of time.
My next step is to dig into the new Learn section when I have the time and energy 
What is the biggest win for you in Coda?
Modern ground-up redesign of Office-like documents; not merely “yet another spreadsheet tool”.
I realized it while browsing the template gallery with examples of in-text computations within a well-organized document containing both types of documents usually tied together in a project: the written part in “Word”, and the data&intelligence part in “Excel”.
Also, all the bells&whistles that make a difference; for instance, formulas like “bulletedList”.
Special mention to the marketing effort to make it used and continuously enhanced.
What feature/concept/mindframe was your tipping point to “getting it”?
Besides getting the gist of it pretty much right away, I think the “aaah” moment was when I “felt” what Coda’s novelty was, in that it allows for a smoother and higher-level usage; an average person with no knowledge of programming or spreadsheets would feel much more powerful with Coda than with Excel, because Coda may require less “logical cleverness” or “hands-on programming” from its user.
However I should add that I’m not sure I really “got it” yet, in that I haven’t really made the jump; I started with tutorials, then tried a project roadmap and abandoned it; then puzzles. I would be more into Coda if I had nearby colleagues using it (besides the online community), but I find it very hard to convince people to use it, especially since it requires a Gmail account and Chrome, and I don’t have enough time to train my brain to this new mindset of formulas and “all-in-one document”. I’ve been struggling to move people from Microsoft Office or Libre Office to Google Docs/Sheets/etc. even though I’m really convinced by these Google products and I’ve been using them myself since their inception, both personally and professionally.
EDIT: Here are 2 revelations (see my reply in this other thread) that make me feel like I’m “getting it” much more now 