I had come across this copy to more than one cell issue when I was curious to see the performance myself in making some random numbers and doing some look-ups based on those results which I couldn't do en masse due to this limitation
a few other things to think about implementing
(1) some more formatting/font/point size options
(2) customizable number formatting like excel
(3) functions like hlookup, edate, perhaps even array function like functionality (i.e., a relatively powerful and open ended underused feature in excel due to a combination of difficulty in understanding as well as being very slow calculation-wise)
(4) some minimal vector drawing abilities
(5) it doesn't bother me and it's part of your marketing but starting with "0" may confuse some people (and would break excel compatibility) since excel doesn't do that (though I appreciate indices/etc. do that in "real" programming)
(6) filtering data doesn't include a "contain" or "does not contain" option
(7) perhaps some limited ability to customize the interface and have user defined keyboard shortcuts triggering formatting/etc. type macros/python
(8) some sort of print/formatting to yield output to pdf