I think this is a natural extension to this thread.

If you were designing an introductory game package, the primary purpose of which is introducing new players (and still be fun for old hands to try with newbie friends), what features must it have? What would be nice, but not a requirement?

If it uses anything other than d6's, include dice!  When I got my first D&D box set, those dice were a big part of the magic.

Extended examples of play. Would try to keep the mechanical text light and easy. Keep the page count lean and incude starter scenarios for the GM. Also, explain things that non-gamers might not understand right away.

As Brenden said, it should include copious examples of play that cover various aspects of the game.  Combat examples should include melee, ranged, and magical examples; as well as rogue abilities in combat.

There should be pre-gen characters outlining the basic archetypes, but it also should explain how to create your own character.

There should be a detailed, sample town/base for the beginning GM.  As well as copious adventure hooks related to the base town.

There should be example of play that includes dungeon adventures as well as wilderness adventures.

There should be a few map examples showing different areas of game play; i.e., dungeon map, town map, wilderness map, and campaign map.  There should be guidelines for new GMs on how to develop their own maps in relation the game system.

Lastly, dice and and a few plastic miniatures.

All of the above, plus online support for the game and a section on how to convert everything over to the main game. The introductory games that we consider to be successful all have these.

Advice and support to enlarge the game beyond its initial scope. Show new players and GMs how to go beyond the example characters, adventures and worlds; encourage them to do so.

Most importantly:  sample characters and an introductory adventure that don't suck!

It seems like a lot of games throw in sample material as obligatory grunt work to be churned out.  The sample adventure should be the best material - a showcase full of cool ideas.  Instead, it often seems like an afterthought put in because it has to be there.

Clear, simple, rules.

Clear, simple explanation as to why and how you'd use the rules.

A lot of what's already been said.

Dice
pre-gens
plenty of examples
clear walk throughs (BECMI was great at this)
What you can do with the full version

I'll throw in a solo play module so you can start right away.  Tunnels and Trolls 5.5 did this with Buffalo Castle and it was a nice and easy way to learn by playing.