Does anyone know how to make it so that Word Wrap is OFF by default in Google Spreadsheets?
There is not, at this writing, a global setting that would automatically affect all new spreadsheets that you create.
To turn off word wrap on any sheet, select all the cells of the sheet and select the "Wrap text" option from the toolbar:
Once all cells have word wrap turned off, it will also be true that new rows and columns will have word wrap turned off, because they take their formats from their neighbors.
However, when you cut and paste a range (or drag-move a range), the empty cells left behind revert to the default format. For this reason, I do not cut and paste. Instead I copy and paste, then reselect the original range and backspace or delete to blank values. This leaves the format intact.
As noted above, there is currently no way to change the defaults for this or, apparently, other features like font or font size.
@erel-segal-halevi: Text wrapping is (now) on the Format menu, not just in the menu bar.
Quick key combination to update your entire spreadsheet to Wrap Text:
Ctrl-a Alt-o then w then down-arrow
Select all; Open Text Wrap Submenu; Select Wrap Text (from overflow - the default)