I regularly use gnumeric for my serious spreadsheet work because Excel can't keep up, and I'm always a little surprised when I have to send a spreadsheet to someone and I find yet another thing that Excel doesn't do; I haven't kept a written list but here's a few off the top of my head (of varying levels of seriousness):
- Excel still gets very confused if you have different files with the same filenames in different directories. At one point it would even, if you crashed while editing one `grades.xlsx` file and went to edit a different `grades.xlsx` on restarting, it would restore the new one from the old swap file, silently clobbering data.
- Last I checked, Excel can't do a lot of very basic data graphing (histograms are the ones that I've run into most often).
- Some versions of Excel (the web one, I think) will just silently not format text that is rotated, making some spreadsheets completely illegible
- I got immediately attached to CSE formulas once I discovered them---they do a lot of things I'd always thought I had to build a custom program for---but 90% of the time when I build and debug something in gnumeric with a CSE formula, it works just as I expected based on experience with abstraction and data structures in other languages, but then when I bring it over to Excel to share with other people, one or more of the Excel functions just don't work properly when lifted over arrays. Then I have to go create an explicit area of the sheet (or another sheet) for my intermediate data and copy formulas to make the computation work, ugh. I really want every single function that normally takes non-range arguments and produces a single value to map over a provided range and produce an array when dropped in a CSE formula. (PS to everyone: if you've never crossed paths with "Control-Shift-Enter formulas", look them up and they'll change your life)
Good to know about the feedback box, though.