Hellcow
Adventurer
There's not much I can say without breaking NDA.
What I will say is that just because something is added to Eberron doesn't mean it's the same as it is everywhere else. Let's take Tiamat as an example. In Dragons of Eberron, we gave Tiamat an official place in the world. Now, you could say "Why bother? If I wanted Tiamat in my game, I'd play a setting that already had her." But what we tried to do was to give Tiamat a unique identity within Eberron - something that makes Eberron-Tiamat different from her incarnations in other settings. In Eberron, she is a rakshasa rajah, a demon overlord of the first age. Where other overlords embody fear, discord, war, and death, Tiamat embodies the touch of Khyber within the blood of all dragons - the potential for evil that lurks within. Just as the dragons of Eberron aren't bound by the color-alignment axis, dragons of all colors are vulnerable to her influence - and fear of her power has held the influence of Argonnessen in check through the ages.
Likewise, Eberron has gnolls - but the gnolls of the Znir Pact are as different from the Butcher's Brood as the Sulatar drow are from Lolth's children. The settings may make use of ideas or creatures created for other settings, or for the generic core. But in my opinion, if the designers are doing their job well, this should provide you with new ideas and approaches to these old things. Thus you get the grist mills of Graywall, the shulassakar yuan-ti, and so on - monsters from the core given their own unique spin in Eberron. Ditto for the planes. Thelanis may be Eberron's parallel to the Feywild, but it is still a unique part of Eberron - not simply "And here's the same plane you'll find in every other setting."
What I will say is that just because something is added to Eberron doesn't mean it's the same as it is everywhere else. Let's take Tiamat as an example. In Dragons of Eberron, we gave Tiamat an official place in the world. Now, you could say "Why bother? If I wanted Tiamat in my game, I'd play a setting that already had her." But what we tried to do was to give Tiamat a unique identity within Eberron - something that makes Eberron-Tiamat different from her incarnations in other settings. In Eberron, she is a rakshasa rajah, a demon overlord of the first age. Where other overlords embody fear, discord, war, and death, Tiamat embodies the touch of Khyber within the blood of all dragons - the potential for evil that lurks within. Just as the dragons of Eberron aren't bound by the color-alignment axis, dragons of all colors are vulnerable to her influence - and fear of her power has held the influence of Argonnessen in check through the ages.
Likewise, Eberron has gnolls - but the gnolls of the Znir Pact are as different from the Butcher's Brood as the Sulatar drow are from Lolth's children. The settings may make use of ideas or creatures created for other settings, or for the generic core. But in my opinion, if the designers are doing their job well, this should provide you with new ideas and approaches to these old things. Thus you get the grist mills of Graywall, the shulassakar yuan-ti, and so on - monsters from the core given their own unique spin in Eberron. Ditto for the planes. Thelanis may be Eberron's parallel to the Feywild, but it is still a unique part of Eberron - not simply "And here's the same plane you'll find in every other setting."