Ramping up my reviewing.
Happily playing games for many, many years.
One of the interesting things about AD&D is that - overall - it has a lot of junk in it. Gary Gygax included things that his friends thought would be a good idea, or that he thought was a good idea at the time, but ultimately didn't do that much for the game.
AD&D, as a system, is another step on the development of fantasy roleplaying since the initial publication of Chainmail. Chainmail was the basis for the original D&D books (and you'd have a lot of problems playing oD&D without Chainmail), and the game changed significantly over the next few years with the release of the supplements, in particular that of Greyhawk.
When you look over the development of D&D, you can see AD&D as a compiled D&D + supplements + magazine articles, with some things cleaned up, but with a lot of questionable elements. Original D&D doesn't have an initiative system: it inherits that from Chainmail. A different initiative system was published in Eldritch Wizardry, but Gygax returned to the Chainmail system for the one included in the AD&D DMG... and what a mess he made of explaining it!
The first time a talented developer and rules editor got his hands on the D&D rules we got Tom Moldvay's take on Basic D&D, which remains one of the highlights of early D&D: well explained and consistent throughout, it is a much superior system to the confused mess that is AD&D.
However, Tom Moldvay's Basic D&D does have one thing lacking: the variety of AD&D. Gygax included Elves, Gnomes, Half-Orcs, Dwarves and Halflings as races, and a full ten character classes. Admittedly, there are a couple of problematic classes - I'm looking at the monk in particular - but that variety is greatly appealing to me and, I daresay, my players.
The major element of Tom Moldvay's revision of the rules I expect I'll lift out for my upcoming AD&D campaign is the initiative rules; sorry, Gary: I can't understand what you want to happen with AD&D's initiative. (They're very much like what Magic Realm did, but then with rules that contradict other rules!)
Another interesting rules point I've recently discovered concerns Magic Item saving throws. There are detailed tables in the AD&D DMG listing the saving throws, but the notes about when to use the table are rather vague. Do you do it every time a PC is hit by a fireball or similar attack? Hit by a giant? Many DMs played it that way, and so AD&D magic items would often have a very limited lifespan. Original D&D provides the original intent: saving throws for items were made only if they were unattended (thus a fireball hitting a dragon's hoard) or if the owner was killed by the attack. That makes a lot more sense to me (and cuts down on the rolling), so I'll be adopting that.
Diaglo, a long-time member of EN World and other boards, often said that Supplement I: Greyhawk ruined D&D. As I become older, I become more sympathetic to that view. On thing in particular stands out to me: ability score bonuses. They cause ability score inflation more than anything else and - in their 3E and 4E versions - give the most balance problems of any element of D&D. Consider their effect on 4E defenses: at high levels, you can be looking at a gap of 9 in the bonus, which obviously translates to a big swing in effectiveness. (10 in one stat, 28 in another).
The original D&D gave a bonus or penalty to XP gained for your prime ability score, and a +1 bonus to missile attacks or hit points for the only other stats that did anything: Dex and Con. Oh, and Charisma influenced henchmen. When Supplement I came along, a 1st level fighter - with very lucky rolls - could get a +3 to hit and a +6 to damage compared to another 1st level fighter with a good (15) Strength. Hmm.
I'm very tempted to return to the original D&D's way of handling ability scores, but it's the one change to the AD&D system that I'm really not sure about yet. I'll work out more tweaks as I get closer to actually running the campaign.