No offense meant here, and I do wish Clarke all the luck in the world with the project, but when I heard about this project the other day I kinda scratched my head. If I want to play a game with the old school feel I'll go grab my old edition books. I like 4E because of what it is and the way it is, and don't really want it to be old school.
First, keep in mind that 1e is a big inspiration for the 4e designers. Mearls said his favorite book is the 1e Fiend Folio, and it's clear that he and Noonan tried to capture some of the essence of those monsters and the "exception-based" nature that they operated under. So, 4e is all ready a little like 1e.
Next, it's not so much about "Making 1e compatible with 4e", as it is having a certain level of Tone. Adventures that are a lot about just exploration, or going into a dungeon with the expressed intent of
killing everything in there. And, 1e had very, very crazy things in it.
I'll use an example. The module "Pyramid of Shadows". It's a dungeon with radically different environments and thematic rooms, all in one place. It doesn't make any sense from a simulationist perspective. It's actually rather wacky. The module, however, "feels" like a 1e adventure
because of that. Another example would be traps that invoke the "Grimtooth" feel.
Additionally, it looks like it's more a matter of taste. For instance, Paladin powers that target
Evil opponents. Succubi that are demons, not devils. Things like that which are more sacred cows that got slaughtered.
Finally, just because people liked the nostalgia of 1e and the feelings it invoked doesn't mean they want to
play with those rules. It's like saying that you want a car that makes you
feel like your first car did, without going out and looking for a 1968 model car; cars that old have massive flaws, if for no other reason than the passage of time.
Granted, I say this as someone who agrees - if you want a playstyle that a system doesn't facilitate, you're better off playing that system you LIKE as opposed to trying to take a crowbar to the other one. And, I took a look at the various suggested fixes Orcus posted, and I want to run far, far away from any discussion like that, because it raises my hackles ("Alignment-based powers, IN MY 4e? No!")