1. Strongly class/archetype based*
2. The "implied setting" with humanocentric focus, etc.
3. Strong focus on the dungeon
4. Gygaxian prose
5. Art (some stinkers, of course, but many very evocative pieces from a number of artists
6. Different, but usually dead-simple, subsystems to fit different concepts.
7. Embrace of randomness
8. Not PC.
9. "Vancian" magic
10. DMG tables (encounters, dressing, tricks/traps, etc)
11. Simple stat-blocks
12. Embrace of resource-management
13. Embrace of player skill as a big factor in success, in addition to PC qualities
14. XP for gold
All of this added up to a game that was extremely well-suited for the kind of play that I enjoy: dungeon-focused, fast moving, challenging for the player as well as the PC, human-focused, et cetera. Note that late 1E (and even some of the original 1E stuff that was commonly ignored) eroded some of this.
* - pre NWP, of course.