It is definitely encourage to see that, for the first time, major upgrade between major CentOS is possible now.
The install media is splited such that it comes with the window managers. For server installation, you should really go using net install.
These are some major changes which should be noticeable. Like Wow!
- Kernel updated to 3.10.0
- Support for Linux Containers
- Open VMware Tools and 3D graphics drivers out of the box
- OpenJDK-7 as default JDK
- In Place Upgrade from 6.5 to 7.0 (as already mentioned)
- Switch to systemd, firewalld and GRUB2
- XFS as default file system
- iSCSI and FCoE in kernel space
- Support for PTPv2
- Support for 40G Ethernet Cards
- Supports installations in UEFI Secure Boot mode on compatible hardware
There are some known issues which you should really consider if you are doing upgrade and make sure you are well prepare.
network - Many people have complained that Ethernet interfaces are not started with the new default NetworkManager tool/have to be explicitly enabled during installation. See CentOS-7 FAQ#2.
installer memory usage - The installer needs at least 406MB of memory to work. On systems with less memory then 406MB the installation will terminate with a fatal error. 512MB is the minimum memory requirement for CentOS-7.
small screen - If your screen resolution is 800x600 or lower, parts of the images shown at the bottom during install are clipped. So watch up on the next,back and cancel buttons.
So I think this is a brief summary to get you started but you can find more at here.