A friend has hooked me up with an advance copy of the AD&D 4th Edition rules and I am very impressed. We've just spent twelve hours or so doing back-to-back dungeon crawls with an accelerated levelling curve and it was a blast.
You need to know that AD&D still does not support roleplaying. I'm happy to say that it does not specifically prevent roleplaying. I guess. I mean, it's not like it will kill your family if you try. But 4th Edition knows that dungeon crawling is its core territory and it zeroes in on this concept like a fundamentalist sniper with a bead on a presidential candidate.
Roughly three quarters of the core ruleset is about combat in one way or another. If your game isn't stuffed full of delicious combat goodness then 4th Edition has little to offer you. Luckily, combat is so indecently entertaining that it feels strange to do it with your clothes still on. You'll need to be playing on a grid with miniatures or tokens to get the full effect, but once you're geared up then it's like someone turned on the "fun" tap and just let it start flooding your bathroom. Each and every class is now balanced for combat, including Clerics, and you'll be happy as a pig in mud whether you're cracking heads with a Rogue, Wizard, or even a Warlord.
Warlords, by the way, are one of the new core classes. What's happened here is that the core party roles have been split up. Melee DPS comes in a bunch of forms: the traditional fighter now appears to be an area-effect melee guy, while the paladin provides focused tanking, the rogue provides focused burst melee, and the warlord offers melee-based tactical support. Spellcasters get an at-will ranged magic attack from level 1, so running out of spells no longer gimps robe-wearers. Warlocks focus on targeted damage, teleportation and battlefield finessing, while Wizards are your traditional glass-cannon nukers.
The entire healing system has been reworked, with healing based off a supply of "healing surges" that each character possesses. Healing spells and abilities allow characters to consume one of these surges, which will heal a character for a quarter of their max HP. Almost every class has some kind of conditional healing ability, which means you can get down to the serious business of murdering goblins without having to drag around a preachy dedicated healer. I think it's a positive step forward for everyone; that whole co-dependant relationship Clerics used to have with other adventurers is a thing of the past, and priestly-types now get to show that they can throw down just as well as anyone else in a dungeon-based streetfight.
The races have also been changed, for no obvious reason. Half-orcs and gnomes are gone; high elves are now called Eladrin (and have an at-will blink power). Half-demon Tieflings are in, as are Dragonkin. While I'm loving the class changes, these race shifts are just bewildering and take away a bit of the AD&D flavour. It's like waking up and finding a mysterious "Uncle Bob" suddenly living in your house and everyone pretending like he's always been there.
Also the old nine-point alignment system is out the window; characters may now only choose from Good, Lawful Good, Evil, Chaotic Evil, and Unaligned.
If what you're looking for is a system in which to tell a story, then in the name of sweet zombie Jeebers try any other system but this. But if what you're here for is levelling up and looting then no-one does it better than AD&D, and now AD&D is doing it better than ever.