They replaced Dalvik with ART http://en.wikipedia.org/wiki/Android_Runtime though it's backwards compatible so your old apps will still work, well, any non root apps. One major issue is with ART it's easier for manufacturers to lock down a device with SEAndroid and prevent writing to /system (rooting) or to block apps from using SuperSU. Obviously you get around this by building your own platform and SELinux policy from Android or Cyanogenmod source, or installing a prebuilt ROM but now it's becoming a requirement to buy phones who's bootloaders can be unlocked (Nexus, Motorola G, ect) because very soon all new phones coming out will be extremely difficult to flash with new software.