Heraldry - I can't remember if every house has an explanation but Tharashk & Lyrandar definetly do. I've only read it once through since buying it so there's probably loads more that I'm forgetting.
But how's about those PrCs!!! Black Dog & Nosomatic Chirugeon kick some.
I can see my next eberron character being a Marked Jorasco rogue/cleric (traveller) with a pre-occupation for diseases and killing the infirm.
Seriously though Nosomatic Chirugeon is the stand out PrC for me in that book.
And the aberrant feats - my aberrant marked barbarian is going to be getting some serious loving!!!
Good work to all the authors, I#'ve been staving off the Eberron books for a while as they weren't up to the quility that I wanted. But Faiths and Dragonmarked seem to me to be where the settings been around long enough so all the authors (not just the might Hellcow) have a good feeling for the setting.
I vely impressed.