I just installed the latest Build of the Windows 10 Technical Preview. The installation took some time and it would be very helpful if the download interface would give some estimate on how long it would actually take - download size was about 2.5GB - it really seems this is a complete re-install, not so much an update. Also the installation process and first logon look more like a migration than an update. Some settings were reset and not carried over (e.g. IE Trusted Sites).
There are a couple of new features, immediatly visible the notification center (which is right now just a plain white windoe which displays notification as they come in). Functionality is off course still missing, the items that are displayed are mostly not actionable - but I am sure this will get better with newer builds.
Windows are now more animated when opened and closed, Storage Sense and Data Sense are here (if you use Windows Phone you already know these).
Most importantly the bugs I have seen in the first release are gone (search from the IE adress bar works again - before I had to first go to google and then search from there).
Obviously there are more changes and tweaks, for example if you click on the network connection icon in the taskbar instead of just a popup on the right you get taken into fullscreen view of PC Settings - not a step forward in my view - but more consistant.
I am sure I will find more changes that I'll write up in a future post - so far, still very stable and usable even as my primary system.