Or, rather, Excel's designers spent very little time and effort into support for non-trivial formula creation and use.
But, if we all seem to agree that Excel is not the place for serious calculations, can we fairly continue to take shots at Excel for being bad at supporting them?
i.e. When I see people maintain non-trivial web apps by opening notepad against production code, it never occurs to me to complain that notepad should have syntax highlighting, collaboration and version management, etc.
But with something like making the formula editor reasonable, I think they do have some blame. Excel costs a heck of a lot more than notepad and it is billed as being a serious tool for professionals to use.
Google Spreadsheets is catching up fast (and has even surpassed Excel in quite a few areas), I'm hoping that this motivates MS to make more improvements.
Admittedly, Excel has a smoother editing experience (which is the real issue here) and is probably better suited for more resource heavy tasks, but Google Spreadsheets excels [intentional] in the following areas:
API -- http://code.google.com/apis/spreadsheets/
Scripts/Templates -- http://code.google.com/googleapps/appsscript/
Forms -- http://docs.google.com/support/bin/answer.py?hl=en&answe...
External data functions -- http://docs.google.com/support/bin/answer.py?hl=en&answe...
Revision History --
Some of this might be achievable in Excel, but it's not integrated as well and requires much more wrangling (e.g. VB scripts).
And yeah, I understand that spreadsheets were never designed with the idea that people who would use them to enter long complex formulas or for purposes where defect detection would be really important, but that's how they're used in the real world.