Original questions posted here.
(1). Race (Elf, Dwarf, Halfling) as a class? Yes or no?
In most games I run, yes, that's the normal set-up. However, I would be open to accommodating someone that really wanted to play a Dwarf Druid or whatever.
(2). Do demi-humans have souls?
Souls are not explicitly discussed in the rules I tend to favor (BX). However, the Raise Dead spell does not distinguish between humans and demi-humans, so I think the answer is "yes". (Note that the Reincarnation spell suggests that at least some members of the following species also have souls: Gnomes, Neanderthals, Blink Dogs, Pegasi, Unicorns, Rocs, Pixies, Sprites, Rock Baboons, Lizard Men, Apes, Centaurs, Werebears, Kobolds, Goblins, Orcs, Hobgoblins, Gnolls, Bugbears, Wererats, Ogres, Werewolves and Minotaurs.)
(3). Ascending or descending armor class?
Descending. Sometimes I'm a grumpy old man who likes things the way they used to be.
(4). Demi-human level limits?
Yep, but then I also assume human level limits. For most campaigns the limit is either 20th (per the Field Guide to Encounters) or 14th (the upper limit of Expert D&D). Not that it comes up much. I usually run lower level games.
(5). Should thief be a class?
Yes, but it is often gimped by harsh rules and/or adjudicated poorly by the DM, myself included. Opening Locks should allow one to earn another roll by continuing to work on it (and maybe giving the DM an additional Wandering Monster check). Finding and Removing Traps rolls should be the last resort of a lazy thief; good descriptions of precautions should preempt the need for a roll. Picking Pockets and Moving Silently should be much easier if the thief can arrange a distraction. Climbing Walls should only be rolled when the obstacle is something no ordinary person can climb. Hiding in Shadows should only be rolled if there's no actual intervening cover. Hear Noise is okay, I guess.
(6). Do characters get non-weapon skills?
Yes and no. Beyond thief skills I don't use any skill mechanics, but if someone said to me "my halfling is a decent cook" I would take that seriously.
(7). Are magic-users more powerful than fighters (and, if yes, what level do they take the lead)?
Yes. If you keep your adventure tempo low they are always better than fighters, assuming one encounter per day and a Sleep spell. But by fifth level the fireballs come out and everyone knows who the boss is.
(8). Do you use alignment languages?
I usually don't. But I think I can make it work in threefold alignment games, using them as religious languages for the churches of Law, the covens of Chaos and the druids of Neutrality. In five- or ninefold games it's just ridiculous.
(9). XP for gold, or XP for objectives (thieves disarming traps, etc...)?
XP for monsters (100 times hit dice overcome), XP for treasure (1 per GP) and XP for carousing (typically 100-600 per drunken bender).
(10). Which is the best edition; ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E ADD, 2E ADD, 3E ADD, 4E ADD, Next ?
I prefer Moldvay Basic and Cook/Marsh Expert, but I do not claim that is an objective choice. Anyone claiming any edition is objectively superior to another is probably either a shill or a patsy.
Bonus Question: Unified XP level tables or individual XP level tables for each class?
XP chart by class, please. I'm all in favor of crummy classes and anyone who plays one should get a bit of an XP aid for doing so.