A day is currently a line in the Gantt chart, but I think it would be far more helpful if a day was a block.

When I enter in the main date range for a task list, starting on a Monday and finishing on a Friday, it looks like the date range is from a Monday to a Thursday. It's great that you can drag the ends of the line on the chart to adjust this date range in a visual way, it's just that the visuals don't quite tie up with the data here.

Look at the month view in iCal – a day is a block and not a line. Enter in an 'all day event' which spans a few days and iCal shows a block over the days you specified. It'd be great if your Gantt chart worked like this too.