First off great plugin! Nice and simple.
I'm on version 2.1.2, and there are 2 hopefully small fixes to the CSS file reference that would improve things.
1) For SSL sites, the straight reference to "http://" instead of of being "protocol agnostic" is throwing an error on my SSL-enabled pages.
Here's from the source of an "https://" page of mine:
<link rel="stylesheet" href="http://MySite.com/wp-content/plugins/easy-columns/css/easy-columns.css" type="text/css" media="screen, projection" />
The fix is pretty easy. When you enqueue the css file you can use the plugins_url() function, like this:
wp_enqueue_style( 'myPluginStylesheet', plugins_url('stylesheet.css', __FILE__) );
2) I am seeing the CSS file referenced on pages that aren't using the columns shortcodes at all. Maybe you could do a check on the page before referencing the CSS file?