Favorite thing...
Hm, that's the toughest question so far.
I like a lot of things very much, but especially the background information on the 4 Eberron-specific races. Particularly the changeling chapter, which details most things from the perspective of each of the 3 changelings philosophies on life as a changeling.
Next in line are the tactical feats and racial substitution levels. They're both very flavorful and interesting IMO.
Random other things I like much:
The fact that the
other races have adventure ideas for the race in question at the end of their brief articles.
Feats -
Aerenal Beastmaster. I've always wondered what the baboon was doing in the Undying Court picture and now I can imagine it - and even get one of my own.

The changeling-specific feats. Very nice.
The kalashtar and psionic feats.
Several shifter feats, e.g., Dreamsight Elite.
Some warforged feats, especially Silver Tracery. (Natural attacks count as silver.) I want a warforged of the Silver Flame lycanthrope hunter now.

Again, the tactical feats. Very impressive.
Prestige classes -
Atavist.
Cabinet Trickster.
Quori Nightmare.
The others are good as well, though.
The shifter-specific magic items.
The vestment of many styles. Many changelings will simply
need this one.
The Truth domain. New options for the Silver Flame are always fine by me.
The concept of mindset spells.
Some of the spells and infusions are nice as well, but I have only skimmed over most of them so far. Insidious insight, insidious suggestion, unseen crafter and dominate living construct look good, though.