Since WotC has seen fit to reprint one of the better editions of D&D (sure, it's limited, and sure, you can only get it in North America [UPDATE: Turns out that's not true], but it's a start), I'm preparing myself to run a campaign using those rules. I'll be using the Black Blood of the Earth setting assumptions, building most of the details during play. Because I am masochistic, I guess, I'll be trying to use as many of the rules straight from the books as possible. I figure that most players, at this point (including myself), have played more complex games, everything from Shadowrun to GURPS, and much more besides, so this shouldn't be too difficult in comparison.
Nevertheless, I'm going to have to house rule some things. Unarmed combat? Using System I from Unearthed Arcana, which was Gary's preferred method, anyway (though no other rules from that supplement; magic items and spells, however, may show up, and I'll take anything from the extended weapon list that I feel like). Encumbrance? I'm not sure, but I may go to Encumbrance by Stone, since Delta is right, and very few people enjoy calculating encumbrance by coin weight.
Weapon speed factors? I want to use these, but even Gary decried them (thanks, Beedo, for that link). I'm still mulling this one over. I've heard a number of options which make them useful, but not overwhelming. On the other hand, I will definitely be using weapon AC "to hit" adjustments. They're really not that complicated in practice (though as a Referee, I'd have to rule on the AC type of monsters or just use the overarching ruling that there is no adjustment for natural armor, which latter I'd feel uncomfortable with, personally), and even easier if the character sheet has space to record them. In both cases, these are elements that make one weapon different than another, forcing trade-offs when choosing between them. This is a good thing in my book. Similarly, weapon length is valuable for charges (and, oh yes, I will be using the charging rules), and space required is also necessary in those narrow dungeon corridors. No way will two halbardiers or doppelsoldners fit in a 10' corridor! Firing weapons into melee and helmets are definitely going to be used. I'm on the fence about random melee targets, though. Also on the fence about morale: I'm going to use it in some form, but whether to use the one in AD&D, the version in Battlesystem, or move to the version in BECMI/Cyclopedia is still an open question.
I'm basically going to stick with segments in some form (we'll see how those weapon speed factors turn out), so spell casting times will be in force. Material components? Track that resource, mages!
Gender-based stat limits are going away. They had some funny ideas of how to do things in the '70s, but that one is kinda offensive in retrospect. Level limits and racial stat limits, though, will be enforced because I like humans and want them at the center of the game. Don't like it? Don't play a nonhuman. I might, depending on how I feel at the time, even disallow nonhuman PCs. That's actually a tempting idea. It would allow me to take the faery races and make them more… fey or whatever. Unhuman. Alignment, on the other hand, sucks rocks as written, and will be definitely jettisoned in favor of a system based on the article "For King And Country" from Dragon #101, or possibly a simple Law/Neutral/Chaos version like the one from classic D&D.
Training times for leveling up will be in force, but the costs will be substantially revised. XP for magic items is definitely one to use.
Psionics and the Monk don't need much modification (well, I might use -C's psionics rules, or perhaps the minor changes in Dragon #78; either way, I'll be adding a Psionicist class). The Bard, on the other hand, will be completely revised as a normal class instead of a proto-prestige class, based on information in Dragon #56 (with the "Songs Instead of Spells" article and original spell progression replacing the spell list in that article).
So, the list of classes will include: Cleric, Druid, Fighter, Paladin, Ranger, Magic User, Illusionist, Thief, Assassin, Monk, Bard, Psionicist. I may include some other classes from Dragon magazine (The Bandit from Dragon #63 is high on my list of consideration), but probably not many.
So, my main area of concern still is the use of weapon speeds. Suggestions for ways to make use of them would be appreciated.
Edit to add: Here's a thought for weapon speed and initiative. Roll initiative by side, but individuals act on a segment equal to their weapon speed modified by the losing initiative roll (alternately, the difference between the initiative rolls) and Dex reaction adjustment. Add segments if on the losing side, subtract them for those who win initiative. So, if the two sides roll 4 and 2, the side rolling 2 adds 2 to weapon speeds to determine segment of action (natural weapons are mostly speed 1), the side rolling 4 subtracts 2 from weapon speed. Results below 1 act on segment 1, above 10 on segment 10. Casting time in segments is the "weapon speed" of spellcasting (and is not modified by Dex reaction adjustment  or winning initiative roll, only modified when initiative is lost [end edit]). Tied initiative rolls act on weapon speed modified only by Dex reaction adjustment. Missile weapons are treated as speed 5 for those which get a single attack in the round; 3 and 7 for two attacks; 2, 6, and 8 for 3 attacks. All attacks from an attack routine go off at the same time. Multiple attack routines happen after a two segment wait (so, if two attack routines start on segment 7, the second will be on segment 9; if one is on segment 9 or 10, the next will go off at segment 10). If two attacks happen on the same segment, the winning initiative side goes first (if also tied, the attacks go off simultaneously). The Dex reaction adjustment is always subtracted (so a Dex 16 subtracts one segment, a Dex 4 adds 2 segments).
Further Edit: I forgot to mention that I am also thinking about modifying the weapon proficiency rules along the lines of the article "Weapons Wear Out, Skills Don't" in Dragon #65. Basically, this groups similar weapons into a single proficiency and adds some formation and special skills (similar in some ways to Feats). Also, I plan on allowing any character to use any weapon at any time, but they can only take proficiency in the listed weapons.