It is annoying to change the start- / end- dates for activities in the plan view - reason is that the duration remains always the same!
If you change the end date (e.g. 10 days earlier) the duration keeps the same and changes the start date as well. Why?
This is just stupid because that's mostly not what I'd like to do.
It would be favourable to set start and end date directly - or via one date and the duration.
You could improve this with a radio button and the user can choose which way to go.
A tip for you could be to use the drag and drop function straight in the gantt view instead. If you hover with the mouse over the edge of an activity you get a little drag area that you can use to change the duration. Let me know if this is good enough for you:)