I heartily agree with everyone's excellent points!
For me also, I really like that Eberron is morally grey and even evil has its place in society, within reason. I really appreciate that in Eberron, nothing is presented as objectively good or evil - each nation, culture, race, religion is presented as having many layered facets. "Evil" nations can pursue goodly objectives to accentuate their evil deeds, and "Goodly" nations can enforce evil actions to maintain altruistic ends.
I really like the fact religions in Eberron, while widespread and monumental pillars of society, are also deeply personal and based in faith and belief rather than any concrete manifestation of a deity. You can't prove the existence of the gods in Eberron and so personal faith is deeply important.
I also really like Eberron's deeply rooted sensibilities in the pulp and noir genre, and its leanings towards the late-19th and early-20th century. This is makes for more a fantasypunk setting rather the traditional high fantasy of the Forgotten Realms, the grim fantasy of Greyhawk, the post-apocalypse of Dark Sun, and the gothic horror of Ravenloft.