Every basic/intro set for D&D released post-1983 has been terrible, and none of them have sold as well as the sets from 1977-78, 1981, or 1983. There's a lesson there. An intro set that tries to reduce D&D to a glorified board-game and doesn't replicate the play-experience of the actual game is worthless. A $25 set that you get 2-3 hours of play from (with little-to-no replay value) before being expected to shell out another $90 for the "real" rules is an insult, and is going to turn off far more people than it turns on. Even a set that covers levels 1-3 like the old Basic Sets isn't good enough nowadays, because it took a lot longer to get to level 4 in those days than it does now (the Holmes-edit (1977-78) Basic Set suggests that it should take from 6-12 sessions for each level gained). To be honestly useful, a 4E Basic Set should probably cover levels 1-10, the entire Heroic tier.
The D&D Basic/Intro Set should be designed from the perspective that most of the people who buy it are never going to buy the full version of the game, but that they should be able to have an enjoyable and playable (and re-playable) game nonetheless. The idea that playing D&D requires a regular investment of 4 hours a week for a year or more plus hundreds of dollars of books and accessories should be thrown out the window, at least for this set. This version of the game should be playable with a random assortment of people more-or-less straight out of the box. It should be possible to go from opening the box to completing an adventure (including character creation and DM pre-game prep) in 2 hours.
The D&D Basic/Intro set should allow new players in a group to start playing within 5 minutes (via a 1-2 page cheat-sheet summary of the rules) with a created character, not a pre-gen -- one of the biggest appeals of playing D&D as a newbie is creating your own character; forcing new players to use pre-gens completely defeats that purpose. It should also include step-by-step instructions to teach the game to new players without a group, but it shouldn't be expected/required for players to read the entire rulebook if they've got someone experienced on-hand to guide them.
The D&D Basic/Intro set should primarily be focused on teaching newbie DMs how to design adventures and run the game. The DM is what sets D&D apart from CRPGs and MMOs, and rather than being the game's biggest liability is its biggest strength. But training new DMs is hard. The Mentzer-edit (1983) Basic Set does probably the best job of this of any version, but there's room for even more improvement. Ideally this set (combined, presumably, with web-tools available on DDI) should allow a DM to create an adventure with a minimum of prepwork - mapping tools and pregenerated maps, pregenrated monster stats and treasure hoards (or tools for automatically generating same), big lists of idea-fodder (tricks and traps, environments, plots, patrons, antagonists, etc.). With a map, a set of monster/treasure stats, and a few ideas picked (or even rolled) from the "idea database" even a novice DM should be able to run a satisfying adventure with no more than 15 minutes prepwork, and this set should teach him how to do it.