Add Field Display Mask
It would be helpful to be able to add a display mask (especially to numeric and date fields) so that you can set how many decimal points show, if commas show or not, specific date formates, etc. It would be most helpful to allow you to create your own custom masks, but you could also supply a list of common masks like money ($0,000.00 at least in US), percent (*100 and display w/ %), zip code (00000-0000 again in US), social security or tax id number (000-00-0000 and 00-0000000 respectively), long date (dd/mm/yyyy hh:mm:ss), short date (d/m/yy), custom date (i.e. 3-Mar-09, 2009-03-03, March 3, 2009, etc) and I'm sure there are others. Open Excel, "format cell" and look at the "number" tab for many examples. I think that this would allow for a greater flexibility without creating a lot of new field types.
CJ Technology Solutions commented
Custom masks would be awesome. For more advanced users it would make the app even more flexible. I think that for more novice users this feature would be useless without providing a pick list of common masks.