Just a revisit!
Joomla devs remained stubborn about adding various useful methods to the core (custom fields, tags) for way too long, so SoftOne and many other previous Joomla users now use WordPress. Joomla still lacks a sensible user-friendly media manager and WYSIWYG editor. Tags, CCK and intuitive media manager functions are primarily what made WP very attractive to end users and developers.
The SEF method of Joomla needs improvement. A simple user should not have to invest in a complex third party extension such as SH404sef just to get proper URLs to grow their SEO. A page with a menu item at site/the-great-page, can still be sourced as site/component/k2/item/34 and site/index.php?option=com_k2&view=item&layout=item&id=34&Itemid=134. This should not occur. The core SEF system should rewrite all URLs to the page as site/the-great-page.
Additionally, each major Joomla version change came with absolute chaos. Many extensions failed, websites broke due to database incompatibility, and the owners had to spend significant time or money to correct errors or perform complex upgrades. Now it seems similar problems are looming with version 4.
If not for the mighty K2, maybe 30% of Joomla users would have switched to another system.