Since most Joomla installations the mootools framework is already integrated, using mootools for the sliding effect and hiding the captcha after sliding would be much more user friendly and traffic saving, especially for people using only this extension by greatjoomla.com or having slow internet connection.
Furthermore all text seems to be hardcoded. Using language files should be a better way, especially for multilangual sites.
One last functional comment: this sliding bar reminds me of the iphone locking mechanism. But this plugin has only 2 steps: left or right. For a better wow-effect it should be possible to continuously slide the bar from left to right ;-)
Hi, jQuery is necessary library to run this extension and usually is compressed by your browser for a next usage. jQuery has 70,4 kB and jQuery UI (for effects like slider) contains 202 kB. Even Joomla! chose Mootools we're staying with jQuery.
Translation is stored in custom file, en-GB.plg_system_cdcaptcha.ini - there is no hard code and you can use this file for multilanguage sites.
About the slide steps we had to change this movement to right-left direction. Some users reported the difficulties with movement on the right side (for example on touch mobile devices).