I initially bought into Eberron because it was a campaign setting that would see continued support, assumes or offers a place for all WotC 3.5 rules and supplements, and allowed me to be there from the ground up rather than laden with years of rules revisions and setting histories. Additionally, it is an easier buy-in for players new to D&D altogether.
I voted Eberron because of it's expansive design. It deliberately avoids excessive detail for the sake of making the campaign your own; I still have tons of ideas for campaigns I'd like to run. Even when I'm not DMing, reading articles and sourcebooks about the world is actually interesting to me, which I can't say for most settings I've explored.
Honestly, I was considering Pathfinder out of curiosity, but I was reluctant to forgo the investment in time, interest and money in Eberron. I liked Eberron and had no reason to leave it behind, but I also didn't want to shoehorn Pathfinder's unique components into Eberron. [Note: According to every setting publisher, their setting is compatible with other campaign settings; I've rarely found this to be wholly true.] Additionally, Pathfinder wasn't a setting yet, it was just a series of modules, and I hate nothing more than having to glean bits and pieces of a world from a story or adventure module. Give me the whole setting all at once, not in chunks of TBD material.
Regarding Eberron's uniqueness among d20 settings, I like the modern analogies it presents. It really helps to open the doors and invite players new to RPGs by letting them do things with which they are familiar, and it allows them to roleplay with a tone that they can mimic from their favorite movies whether they are modern, sci-fi, fantasy or whatever.
I also like the static timeline of Eberron. Limiting advancements and drastic changes to the world or events means no arguments over canon and what happens in my Eberron.
The interestingly unique interpretations of the core races is a favorite of mine, too. It really brings a lot of value to the races and the setting as a whole, although I'm somewhat disappointed that dwarves don't have more to them; they seem pretty standard unless I'm missing something.
I like the logical progression of magic as a resource rather than a kept secret only used by the elite snobs. Magic can make everyone's lives easier, so why not use it to the benefit of all. In fact, I like that there are modern-like wonders without the waste produced from using energy sources that are harmful to the environment or limited in availability. Even the production of lumber can be magically aided.
Monstrous races as citizens of the world rather than things to kill gives players something to think about before slaying a hobgoblin, gnoll or orc.
Last but not least, I love the pulp feel of Eberron. Sure, other settings can accommodate it, but other settings aren't built on it.