Upgraded to 4.6 Civicrm javascript errors Error can't send email or press some buttons
issues.civicrm.org is down right now - but I see this issue: CRM-12570 "Javascript error when mootools is loaded in Joomla backend".  I'll also note that everyone reporting this problem is using Joomla.  So my guess is that there's a Joomla plugin that's causing a jQuery conflict or similar.

To troubleshoot this, I would:


Disable all your other Joomla plugins.  I hope you're doing your upgrade on a dev server before doing it on a live server!  If disabling all of them solves the problem, start turning them back on until you can identify which one has the conflict.
Check your Net tab in your browser's developer tools when loading a page on your site vs. on the demo Joomla site.  Are there any errors, or unusual JavaScript files on your live site compared to the demo?  If so, identify their source.