The official change-log is unavailable at this point but we hope to hear from the CM team soon, which should reveal what has changed in the CM 11.0 since M11.
Update (Official Change-log): CM team has stated that the M12 release focusses on stability and security fixes. It includes Google patches against POODLE vulnerability in SSLv3 as well as Multi-SIM improvements.
To remind you, CM 11.0 is based on Android 4.4 KitKat and the work on Lollipop-based CM release has just started and it will take some time for the Android 5.0-based CM Snapshot builds to arrive for the supported devices. CM team has stated that first nightlies of Android 5.0-based CM 12 can expected later this month or early next monnth.
Here is a look at just some of the popular devices for which CM 11.0 M12 is available: Sony Xperia Z2, Amazon Kindle Fire, HTC One M8, Samsung Galaxy S, Galaxy S II, Nexus S, Nexus 7, Galaxy Nexus, Moto G, Nook HD, Note 3, and more.
If your device had got M11 build but you don’t see the M12 build on CyanogenMod website or in CM updater, it will most probably arrive in the next few days.