The 4E Eberron Player’s Guide has a great section called “Ten Important Facts” that boils down the highlights of the setting into two brief pages. Here’s an even shorter version:

1. If it exists in D&D then it has a place in Eberron.
2. Eberron takes the cinematic action of D&D and adds a strong dose of mystery and scheming.
3. Magic is common enough to allow conveniences and services usually unavailable in traditional medieval fantasy. (Binding elementals to create trains and airships, mail services through scrying, etc.)
4. Eberron is full of exotic, dangerous places to have pulp adventures.
5. The world is emerging from the Last War, which has either ended for good or been put on hold, depending on who you talk to.
6. The Draconic Prophey is as complex and unfathomable as the dragons themselves. The prophecy can be read through the sky’s celestial bodies and through strange phenomenon on land.
7. Now technically four kingdoms after the destruction of Cyre, the Five Nations are the major players in the setting.
8. With the ink still drying on the Treaty of Thronehold, intrigue is everywhere.
9. The Dragonmarked Houses, transcending political boundaries, fundamentally shape life in Khorvaire.
10. Dragonshards are ever-present and are the ultimate “a wizard did it” explanation. They come in three flavors: portentous Siberys shards that fall from the sky, arcane Eberron shards found in the soil, and elemental Khyber shards found in the impossible depths.