Ideally the grid would be able to serve in multiple capacities, but here are some that I think are important:
Excel like functionality with enhanced features. For example, Excel has the ability to select from a domain list, but it is very limited. Having calander controls, drop downs, calcs, etc would be helpful. But maintain the ability to copy, paste, delete just like Excel. Obviously the ability to programatically what can be entered and how it can be edited is important. Basically if each cell is treated like a textbox, but together they are a grid.
It would be nice to be able to create recursive data (multiple values in one cell) and have the grid control handle it in multiple ways. For example you could have cell scroll button, you could have a value or an icon to indicate multiple values, so when they double click the cell they get a small form popup that shows the multiple values. However, the ability to copy and paste multiple values (separated by a "," or ";") and have the control know how to handle this would be a huge benefit. Most grid control handle hierarchy from a standpoint of complete records (ie, show me and group the stores that are located in each state), but very fwe have any elegant way of saying for an attribute named Country of Origin, the values are US; CN; CA and be able to parse out the contents of the cell as opposed to viewing that as a string.
If we could get a control that did these things, our prayers would be essentially answered.