For exactly what you described use MS Word (if you insist on excel, see below):
In MS Word, switching to Outline view (View > Outline) allows you to assign different outline levels for each paragraph ("task"). Create a paragraph at level 1. That's your "Task". Now create more paragraphs but assign them a lower level. These are the tasks items. Word gives you the plus/minus exactly as you described. You could expand/collapse each group, move it up or down and more.
In Excel, you can get plus/minus functionality by using the subtotals tool (not subtotal function). Copy this sample:
Task Subtask Items Details Due By
Task 1 Sub1 Do this asdf 1/1/2010
Task 1 Sub1 Do that asdfw 1/1/2010
Task 1 Sub2 Do this sdfa 1/2/2010
Task 1 Sub2 Do that wef 1/2/2010
Task 2 Sub1 Do this sdfa 1/3/2010
Task 2 Sub1 Do that fasdf 1/3/2010
Task 2 Sub2 Do this ytgj 1/4/2010
Task 2 Sub2 Do that ewrf 1/4/2010
Now use the subtotals tool found in Data > Subtotals... to create a subtotal for each change of value in the Task column. Rerun the Subtotals tool but make sure "Replace current subtotals" is unchecked and specify the Subtask column as the "for each change in" column to add another level. It's ugly, but you could expand/collapse.
If you have Outlook (most office installations include outlook) you could try using the task folder in outlook. You can customize the view, sort the items, filter the list, add attachments, due dates, alarms and much more. I don't know of a way to group things though.