I agree with those saying DnD has the makings of a niche game, and more so with 4E than earlier editions; something much more rules-light would probably be better.
I think it's very common for experienced gamers to think that "rules light" is a good idea for an introductory RPG.
I also think it's very, very wrong.
It may be true for players who are being introduced to the game as part of an experienced group. But if you're talking about a game that's being sold to entirely new players, then the ideal quality for an introductory game is one which can always provide a solid answer to the question, "What am I supposed to be doing?" And it should be able to answer that question for both the GM and the player.
And for that to be true, you need lots of detailed crunch. New players
want the rules to tell them a 20-foot jump is a DC 20 check.
The other quality an introductory game should have is a default scenario structure that, similarly, guides the players and GM in obvious ways. The D&D dungeon crawl is ideal for this: The PCs are in a room. They can either do something in that room or they can take one of (several) obvious exits and go to the next room.
For the players, the dungeon crawl never leaves them wondering, "What should I do next?" And the structure of the dungeon crawl makes it very difficult for a player-proposed action to leave the DM wondering, "What should my response be?" (You look at the map and describe the next room.)
And if this default scenario structure is also extraordinarily easy for the newbie GM to homebrew new scenarios for, then you've
definitely got a winner.
And, finally, the game also needs to be flexible enough to expand beyond that starting point. And broad enough in its appeal to support the interests of many different types of players interested in pursuing many different playing strategies (and I'm not just talking combat strategy here).
In short, I think D&D's success wasn't a fluke. I think it was extraordinarily well-designed to appeal to new players (despite the opaque quality of the original manuals).
A couple final thoughts:
(1) If true storytelling games like 3:16 and Shock: Social Science Fiction ever get their acts together in creating a game structure and rulebook which is friendly to people who are not already intimately familiar with the conceits and forms of interactive fiction tabletop games, they have a real potential to create a breakout title.
(2) If you ever win the lottery and want to create a new RPG giant to compete with D&D, your best shot is to (a) look around and find a popular non-RPG video game property; and (b) license it under the condition that your 24-page introductory rulebook will be distributed in the next version of the game.
If your introductory rulebook is designed as effectively as the Basic D&D boxsets were -- featuring a ruleset friendly to completely new groups of players -- then you'll have introduced yourself to a million+ potential customers. Get a 1% conversion rate and you'll be in good shape. Get a 10% conversion rate and you'll be huge.
Will you ever earn back your investment? Depends on how effective you are in capitalizing in the window of opportunity you'll have opened for yourself.