I think your question is too broad to be answered completely here, but I will answer with a truncated list in terms of 'major' changes taken form the CentOS wiki:
As with every first major Release most of the packages have changed
and have been updated to more recent versions. It would exceed the
scope of this document to list them all. Most notable changes are:
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)
LVM-snapshots with ext4 and XFS
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
For more in-depth information on changes I would suggest reading over the RHEL 7.0 Release notes.