I'm always excited to kick the tires of a new supported release (LTS) of Ubuntu. In most cases I will be using the LTS image for 2-3 years to come, possibly longer. Stateless machines (application server front-ends, in my case) tend to make the upgrade cut pretty quickly. Meanwhile database servers, log servers, and the "who set this up?" servers usually end up on the tail end of any upgrade priority, and require more delicate handling.
I do not use nor am I familiar with, the desktop windowing systems for Ubuntu Server. My use of the OS is limited to bash, so I won't be discussing any UI updates.
Apache 2.4 support
Apache 2.4 has some excellent new features, in addition to better modularity through MPM support. We have been using 2.4 in production environments for several months now, and have seen significant improvements in memory utilization and overall throughput. Apache 2.4 has been available through PPA repositories for a while (in Ubuntu 12.04), but it's nice to finally have official support.
PHP 5.5 support
I use PHP quite a bit in my daily work assignments. The recent increased rate of development has been a welcome change. I'm not going to do a deep-dive concerning the significant improvements to PHP from 5.3 to 5.5, however, having stable access to 5.5 helps move the entire community forward. If you are currently using 5.3 you should start making plans to test your applications in version 5.5. Version 5.3 received end of life cycle status over a year ago.
Node.js 0.10 support
Again, stable access to updated packages by default is a much easier sell than needing to install from a PPA repository. The Node.js platform has changed and matured at a blistering pace. Continued support on Ubuntu has been available through Chris Lea's repository ppa:chris-lea/node.js which provides an outstanding service to the Node.js community.
Though I feel that Juju was a awkward choice for a name (I imagine having to discuss things like this in meetings), I am intrigued by the prospect of deploying an entire stack from Canonical's library of Charms. The idea of leveraging publicly scrutinized open-source scripts for complex services will help organizations better utilize DevOps resources and expertise, and rapidly build out scalable infrastructure.
I am responsible for a MongoDB cluster that could benefit from this "managed services" style approach to implementation and maintenance, while still providing the transparency of building your own deployments. Tools like Juju and AWS CloudFormation provide more control than an auto-magically scaling PaaS options that are often shrouded in a black box with a volume knob on the top.
There are numerous stability and security improvements, along with other updates I did not mention: MySQL, Nginx, Puppet, and PostgreSQL. As package updates outpace stable channel repository support, I will likely continue to use PPA channels to get the latest and greatest when it is practical.