Jack Daniel
Legend
Bring back AD&D in print (both 1st and 2nd), as well as the D&D Rules Cyclopedia.
Since that's not going to happen, we probably ought to be saying, "Make sure that 5th edition feels (and plays) more like these old editions than 4th does." It could happen: 3e did a lot of hearkening back to 1e, flavor-wise, to "undo" what many older gamers saw as mistakes made in 2e. 3e brought back the demons and devils, the monks and half-orcs, the assassins and barbarians.
Of course 2nd edition wasn't a bad game. (It's certainly one of my favorite editions.) But it nevertheless has this unsavory reputation as the watered-down, corporate-shill, Gygax-booting, kiddie-cleanup of 1e. Entirely unjustified IMO, but that was the perception. And 3e handily addressed most of those (shallow and superficial) "mistakes." The train of thought apparently went something like this: "If there are demons and devils back in the MM, the problems are solved and all is forgiven."
So, looking forward to the inevitable 5th edition, what can we count on? WotC created 3e to respond to the problems that people had with 2e (the aforementioned flavor issues being only a small part of that, compared with the major notion that 2e was dated, clunky, and arbitrary, and that 3e would therefore need things like a universal task-res mechanic and ascending AC to bring it up to date). Likewise, 4e seemed at least initially motivated by addressing the problems seen with 3e (bloat, complexity, prep-time, and game imbalance outside of the "sweet spot"). Thus, we can at least expect that the future designers of 5th edition will bend over backwards to correct whatever superficial problems are most vocally attributed to 4th edition (and when the PR campaign comes, they'll naturally go out of their way to explain their goals and demonstrate how they've met them).
Therefore, and I mean this in all seriousness, it's probably the shrillest set of voices on message boards like this one which will be heard and acknowledged when the time comes to create and release 5th edition. So if you want to influence the future of the game, don't clamor for PDFs, because that's pointless. Don't ask for nostalgic releases of out of print editions; it's a pipe-dream. Instead, be clear about why 4th edition doesn't satisfy. Don't quit discussing the relative merits of the various editions of the game. Don't be intimidated into silence at the first alarmist cry of "edition war!" It's the only way we'll ever see a new edition of the game that draws upon all the best aspects of its predecessors.