'Border radius' on a column chart currently affects all 4 corners of a column identically. Please, could we have the ability to set individual corners at different radii?
Personally I would like this so that I can round just the top of a stacked column chart, but i think it would be a great addition and allow some nice extra flexibility in look & feel.
I created a plugin for this feature, see http://www.highcharts.com/plugin-registry/single/5/Rounded-Corners
Edit 2014-05-07: Co-worker Stephane also has a nice drop-in code snippet for creating custom shapes: http://jsfiddle.net/highcharts/My9Qp/