First of all here is a video on Windows 10 for Phones from the official team
The new features includes
Full-size background Image for Start
More quick actions in action center
Better speech to text capability
Better Photos Application
Universal application which will be helpful when you use a windows 10 PC and windows 10 phone
If you are interested in testing technical preview builds, you need to do the following
1. You need to install Windows Insider application, this application is an international one which also supports 25 languages too.
2. Login using Microsoft Account you registered on https://insider.windows.com/
3. If your phone have an update you will get it and will have to set Slow/fast (the speed of updates you need)
4. Make sure there is atleast 40% of battery remaining while the update begins
As of now, you will need to have any of the following devices to get the technical preview
You maybe wondering why many of the high end phones such as Lumia 930 and 1520 are not in the list, Microsoft says
Some context on why we chose these and not higher end phones like the 930/Icon or 1520: We have a feature that will be coming soon called “partition stitching” which will allow us to adjust the OS partition dynamically to create room for the install process to be able to update the OS in-place. Until this comes in, we needed devices which were configured by mobile operators with sufficiently sized OS partitions to allow the in-place upgrade, and many of the bigger phones have very tight OS partitions.
There are known issues for the current Technical preview version so before proceeding please make sure you read all of them and decide whether to proceed or not. There are workarounds for some issues too.
You can read more here on the official Windows blog