One thing that astounds me is the humble spreadsheet. Non-programmer folk build wild and wonderful solutions to real world problems with a simple grid of formula. Replicating their efforts in desktop application often takes 10 to 100 times longer than it took to write the spreadsheet and the resulting application is often harder to use and full of bugs!
I believe the key to the success of the spreadsheet is automatic dependency analysis. If the user of the spreadsheet was forced to use the observer pattern, they'd have no chance of getting it right.
So, the big advance is automatic dependency analysis. Now why hasn't any modern platform (Java, .Net, Web Services) built this into the core of the system? Especially in a day and age of scaling through parallelization - a graph of dependencies leads to parallel recomputation trivially.
Edit: Dang - just checked. VisiCalc was released in 1979 - let's pretend it's a post-1980 invention.
Edit2: Seems that the spreadsheet is already noted by Alan anyway - if the question that bought him to this forum is correct!
