This is going to be a weird and longish post, but I think it still fits the topic, so bear with me, 'kay?
For some years now, I've been running oD&D campaigns almost exclusively. Lots of things about classic D&D appeal to my sensibilities: I like that the lists of monsters, magical items, and spells are comparatively small. It gives the game a sense of finitude and completeness, even if the only rulebooks you have are the Red Box and the Blue Box, or the Rules Cyclopedia. And I really love the class system: I like that the core of the game describes only fighters, clerics, mages, thieves, elves, dwarves, and halflings, and then it leaves the player to interpret things from there. No need for paladins, rangers, barbarians, gladiators, cavaliers, swashbucklers, and samurai; they're all just fighters. There's no finagling with race-class combinations to produce the most specialest snowflake you can: if you're a dwarf, then mechanically you're a Gimli clone, that's the end of it. You have to role-play to make your take on a demi-human character unique.
But I'll happily admit that I'm starting to miss the AD&D way of doing things. Classes; sub-classes that you have to roll good stats to pick; races that can choose from a handful of classes and multi-class combinations. Plus, I got my hands on a copy of the Advanced Edition Companion for Labyrinth Lord, which translates most of the 1st edition classes, races, spells, items, and monsters (it leaves out the freakshow fighter/thief/druid prestige bard class) into the basic D&D rules. So, perhaps, the next campaign I run will be a return to my AD&D days.
Back then, we played 2nd edition. There are lots of things about 2nd edition that are good: bards and rangers spring immediately to mind. Some things, not so much: clerical spheres and wizard schools, mainly. It was definitely better in 1e, when the four major spell-casting classes (cleric, druid, mage, illusionist) each had their own discreet spell list. To that end, I think I'm going to have to try something that I've oft heard about but never actually tried: the infamous 0e/1e/2e mashup campaign. Labyrinth Lord + Advanced Edition Companion + 2nd edition core rulebooks, to be precise.
I miss 2nd edition's system of class groups. Warriors, priests, rogues, wizards, and psionicists. Everything fell into place under one of these groups. I'm inclined to leave out psionics for a couple of reasons, though: the original psionicist was notoriously unbalanced, and I don't care to use the Skills & Powers version; and anyway, I don't see the need to draw a distinction between psychic and magical powers. Wizards already fill the niche; just because they mumble and gesture doesn't make them seem any less "psionic" to my way of thinking. But this consideration does still beg the question: how do you strike the proper balance between too few classes and too many? After all, by the end of 3rd edition's run, there were definitely too many base classes (and 4th edition suffers from the same problem in my opinion). So just what niches are there that need to be filled?
I think, as one might expect, it comes down to archetypes. What kinds of fantasy characters are so broad or so important that we want a whole class to represent them? After thinking about it for a while, I've come to the following conclusions: (1) standard fighter, (2) paladin or crusader, (3) wild warrior, like a ranger or barbarian, (4) priest or cleric, (5) druid or shaman, (6) kung-fu monk, (7) thief or burglar, (8) jack-of-all-trades bard, (9) ninja/spy/assassin, (10) standard mage, for flashy evoc./conj./trans. magic, (11) illusionist/enchanter/beguiler, and (12) necromancer.
Elegantly enough, this breaks down into three classes per group:
WARRIORS: Fighter, Paladin, Ranger.
PRIESTS: Cleric, Druid, Monk.
ROGUES: Thief, Bard, Assassin.
WIZARDS: Mage, Illusionist, Necromancer.
The illusionist, in particular, I've always liked to re-purpose as the "psychic" specialist. They seem to me the most "mentalist" of magic-users, conjuring up their illusions and phantasms by actually playing with the minds and perceptions of their victims. Like "the Shadow" from the old pulps and radio serials. If you've got a dedicated illusionist class in the game, there's no need for psionicists.
Now, as to the races in the game... I've tended in the past to take a pretty kitchen-sink approach. When you have rulebooks like Creature Crucible and Orcs of Thar, with whole classes dedicated to playing pretty much whatever, of course I'm going to allow the players to pick weird races and play them. But I'm starting to miss the simple focus of by-the-book AD&D. I might even turn the dial up to eleven. Humans: a must have. Dwarves: ditto. Do I need both elves and half-elves? Probably not; the vagaries of fantasy genetics aside, I think it's easier to just say that that the mixed children of humans and elves take after one race or the other. (Practically speaking, half-elves always seem to get the short end of the game-mechanical stick, so I'd rather just drop them.)
Then there's gnomes and halflings... I've often heard people on these very boards argue about why they drop one race or the other. "Halflings are too Tolkien." "Gnomes are too DragonLance." Whatever: I love both gnomes and halflings. But there is a valid point here: they both seem to fill the same archetypical niche of "sneaky little people." The natural thing, it seems, is to combine them: call them gnomes (the name caries folkloric weight) and fluff them as hobbits.
Then we come to the bruisers. Half-orcs and half-ogres. I prefer to drop these guys for the same reasons I'd leave out half-elves. I don't want my AD&D campaign to become a smorgasbord of half-breeds, templates, and rejects from a Xanth novel. But I want a bruiser in the lineup. Ogres? Too monstrous. Goliaths? Too artifical. Centaurs? Now we're talking: that's a species with some mythological heft. Plus, hey, I'm a Shining Force fan.
So... the race-class combos. It's kind of a ritual, when coming up with homebrew rules for an AD&D game, to delineate these. (In fact, I think this might be the part I miss the most.) Anyway, here's what I'm thinking now:
HUMANS: Any one class; humans other than paladins, monks, and bards can become dual-classed.
ELVES: Fighter, ranger, cleric, thief, or mage. Elves can be multi-classed with any combination of two or three classes that does not incorporate both cleric and thief, hence: F/C, F/T, F/M, M/T, M/C, F/T/M, and F/C/M (F = fighter *or* ranger).
DWARVES: Fighter, cleric, thief, assassin, or necromancer (to give dwarves a potentially creepy edge). Dwarves can multi-class as F/C or F/T (T = thief *or* assassin).
GNOMES: Fighter, cleric, thief, assassin, or illusionist. Gnomes can multi-class any two of their classes together (except, of course, that thief and assassin are in the same group and can't be combined).
CENTAURS: Fighter, ranger, druid, bard, or mage. Centaurs can multiclass F/D, F/M, or F/D/M (as with elves, ranger can substitute for fighter in these combinations, assuming some relaxed alignment restrictions that allow for good druids, thus making ranger/druid a legal combination).
*Whew*... I can't believe how long that actually turned out. I went from, "hey, I miss this about AD&D, wouldn't it be cool if...?" to "hey, listen to this campaign manifesto I'm writing up..." So... sorry about that.

Anyway, that's what I miss. 2nd editiony race and class stuff. (Wow, that was a lot quicker and easier to type.)