I've often thought about writing up a lite version of the full rules to be an intro set. The srd is very nice, but it is huge and unwieldy. My thoughts were:
Only three races, elves, dwarves, humans.
Only three classes, fighter, rogue, sorcerer.
Don't offer multiclassing.
This provides nine character options and the classes are fairly customizable with feat and spell choices. It also covers fighting, spellcasting, and indiana jones adventuring D&D character niches.
Cut down the spell list to maybe five to ten straightforward spells per level.
Don't offer item creation feats.
Only provide a certain number of monsters so it is not overwhelming.
Healing would have to come from resting and potions you bought at a potion shop. The other options would be allowing cure spells on the sorcerer list but that makes it in conflict with core rules, or to allow clerics, which are very D&D but not as necessary for a fantasy RPG experience.
I've debated whether to keep class vs. cross-class skills or to make everything class skills, probably keep the class skills if it is to work as a compatible intro system. I also debated not including skills at all and give rogues a class ability to detect and disarm traps, open locks, climb, etc. as if they had maxed out their skill selection in these skills. To keep it completely compatible probably best to have full skill rules.
Probably keep combat exactly the same, but maybe not include all the combat options (sundering might go, tripping, bull rushing).
Maybe simplify xp to advance a level every four games.
Probably not include alignment
The idea is to provide functional mechanics but not bury the new player in a blizzard of options and variations while they learn the basic mechanics. It should still have the same fantasy dungeon crawl feel though, it should be a playable game, not just a primer. They should still be able to kill things, take their stuff and wander about a neat, evocative fantasy world.
The skills and intricacies of combat options (especially AoO) would probably be the most complex aspects of the system.
Of course this has not progressed beyond the thinking stage.