First things first, Welcome ^^
The changes are extensive, as Ashes of Erebus is a follow-up to Rise of Erebus which was itself a follow-up to Fall Further, so there's three mods worth of content ^^
An important thing to note is that there are two versions, the Release version and the SVN version. The Release one is generally more stable but has less content and bugfixes than the SVN one which gives you access to the updates as soon as we work on them ^^.
The mod also uses an extensive module system, meaning that some of the content can be deactivated easily.
In term of actual content, you'll find reworks of the core ffh civs, some new civs, new leaders, an improved fort, lair and commander mechanic, and others. Your FFH instincts are still mostly valid though.
Finally, beware of the wild life which is generally harder than ffh. Very early game should be about reinforcing your defences.
If you have any question, i'm available to answer them.