1) Quick to create characters. With a minor amount of experience, players could make a character in 15 minutes or less.
2) Quirky magic items that were not formulaic in any sense. Wands had multiple functions, miscellaneous magic items were often mysterious to figure out, and even mixing two potions or drinking them too many at one time was Russian Roulette. I loved a sword that was +3 to hit, +6 vs. cold creatures, and could extinguish all flames in 10 feet just by thrusting its blade into them, or a sword that could have about 4 different types of bosnuses depending on what kind of reptile you were.
3) The math from 1st to 9th level worked really, really well. Very rarely was a monster so tough that even the Magic-user couldn't hit it with a staff.
4) The DMG had advice, charts and tables running from humorous to truly inspired. I may never use the "Wandering Prostitute" table, but I can still use those random dungeon maps, or dungeon dressing tables, or lycanthropy rules, or rules for hiring common laborers and sages.
5) The strong division between classes and their roles. It's one thing I actually am hopeful about for 4E; in 1E, fighters and wizards and thieves, etc. really didn't step on one another's toes. The magic-user, often touted as making a thief obsolete, really could only pull his tricks a few times per day.
6) Ranger giant-class bonus. YEAH, BABY!
7) The ease of running monsters in that system; about a dozen or less stats and that's it. Again, one of the tricks I'm noticing they're pulling for 4E.
8) Spells that had drawbacks that had to be considered. Compressive Fireballs, rebounding lightning bolts, Fly Spells that could end and drop you out of the sky if you weren't cautious with your time limit, and haste spells that could kill you of system shock if you were unlucky all added to the risks inherent in magic.