I posted a link to Jeffro's blog on G+ the other day. Now Alex Schroeder insists that I shouldn't be allowed to just post stuff, I have to work for it and answer the Trollish Questions myself. Much like he did here and here. Well alright then!
Race (Elf, Dwarf, Halfling) as a class? Yes or no? Depends on the kind of game I run. For my current "Expedition to the Borderlands" open-table game I use B/X as written. I might tweak it if there was a reason for it, i.e. if the players went to the Dwarven Clan in the West and begged to recruit a cleric of Moradin or something. For a "real" campaign, the ominous "home game" I keep dreaming about, I'd go with my own version that splits race and class although the "chassis" of the rules would still be B/X.
Do demi-humans have souls? Does anybody? Under the weirdo D&D-esque assumption that the dead "transcend" to a plane according to their alignment, sure. Whatever it is that "does the transcending" could be called a soul. Doesn't really matter though, I don't run planar games. (Wait, maybe I would. If the only way to get to the planes was if the characters killed themselves, that would be kinda cool.) What matters is that you can resurrect everybody just fine, doesn't matter if they are human, demi-human, humanoid, or intelligent mushroom. And nobody even gets a saving throw against it. :-)
Ascending or descending armor class? I really don't care, plays more or less the same. But Target20 works great with descending AC and I grew up with descending AC and I use resources that have descending AC listed so that's what I use. (I only get annoyed by the silly little difference between D&D and AD&D.)
Demi-human level limits? Sure. It's only fair. Again, it depends on the kind of game I run. In the "home game" I'd use my own version with my own crazy "rationale" behind it all. You should try inventing your own rationale as well!
Should thief be a class? Sure. If you don't like them, don't play one! But anybody can attempt to do what a thief is able to do, they just never get much better at it. Wizards can fight with swords too in my game, they just suck at it so it's a really bad idea for them. And if a halfling decided to cast a spell from scroll? I'd probably give them a chance! A very, very low chance. With hilarious consequences if they mess up.
Do characters get non-weapon skills? Sure. But it's not really formalized. If you want your character to be good at something that's not in the rules, ask me: Maybe I'll let you write something down. I won't let you write things down again and again, not even if you're playing an elf who could have learned 27 different trades by the time they turn 314. (And of course it's easier to convince me that your halfling can cook and that your dwarf is a decent smith than to convince me that your elf is a deadly assassin and that your fighter can fly.)
Are magic-users more powerful than fighters (and, if yes, what level do they take the lead)? Sure. Wait what? Troll-trolly-troll left out thieves and clerics and all the other fun things like Balrogs and Gargantuan Purple Worms. Useless question. Next!
Do you use alignment languages? Not as they are usually defined in D&D. So let's just say "No!" and be done with it.
XP for gold, or XP for objectives (thieves disarming traps, etc…)? I use B/X RAW for the open-table, so XP for gold and some for monsters overcome. I'd probably switch to "XP for money spent" for the "home game" I keep talking about. Sometimes I like "objectives" but they are so anti-sandbox. (I hate AD&D2 in this regard, so that's a total no-no. But I suffer through it for the game I play in.)
Which is the best edition; ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E ADD, 2E ADD, 3E ADD, 4E ADD, Next? I grew up on BECMI and really liked RC for a good long while. But that was because I never knew about B/X, it wasn't available in Germany when I got started. There's no question in my mind that B/X is the best overall package, period. I do like AD&D as well, but only Gary's and only in mild doses. So I guess what I am saying is I tend to like some of the AD&D classes (hence probably my fascination with this thing). AD&D2 is horrible, just horrible. I'd probably play AD&D3 if it's just the core three books, but I wouldn't touch 3.5 or 4. I have no real opinion on 5 yet. I like some mechanics and I appreciate that they rolled some nice options into character generation. But really it's B/X for me. OD&D and Holmes win special prices for being, well, special. But I don't play them.
Unified XP level tables or individual XP level tables for each class? This was Jeffro's bonus question. Again, depends on the game. In my "home game" I would use a unified XP table, the 2000/4000/8000/... Fighter table. Edit: Should have linked to this post.