The way the new protection feature is set up, you don't directly protect a
specific cell or other range. Instead, you assign "editors" who are allowed
to modify certain areas, and any area that doesn't have any editor will be
protected. Although you can specify individual people as editors by
supplying their email addresses, you probably want the Everyone group to be
able to edit the parts outside the protected cell.

As far as I can tell, it isn't possible to assign Everyone as the editor for
the whole document and then delete that editor for the specific cell to be
protected. You must assign the editor for each individual range that is not
the protected cell: the area before the table, the area after the table, all
rows in the table before the cell, all rows after the cell, and the cells in
the same row to the left and right of the protected cell.

The following macro is extremely simplified, as noted in the comment...

Sub ProtectCellOnly()
  ' Oversimplified example:
  ' Assume the document has one table, and you
  ' want to protect the cell in the second row,
  ' second column.
  ' Note: in a real environment you need error-
  ' checking to ensure there is a table and that
  ' it has a .Cell(2,2). The macro will also fail
  ' if the table contains any split cells or
  ' nested tables.

  Dim oRg As Range
  Dim oTbl As Table
  Dim oRow As Row
  Dim oCell As Cell

  Set oTbl = ActiveDocument.Tables(1)

  Set oRg = ActiveDocument.Range
  oRg.End = oTbl.Range.Start ' = doc range before table
  oRg.Editors.Add wdEditorEveryone

  Set oRg = ActiveDocument.Range
  oRg.Start = oTbl.Range.End ' = doc range after table
  oRg.Editors.Add wdEditorEveryone

  For Each oRow In oTbl.Rows
     If oRow.Index <> 2 Then
        oRow.Range.Editors.Add wdEditorEveryone
     Else
        For Each oCell In oRow.Cells
           If oCell.Column.Index <> 2 Then
              oCell.Range.Editors.Add wdEditorEveryone
           End If
        Next oCell
     End If
  Next oRow

  ActiveDocument.Protect Type:=wdAllowOnlyReading
End Sub