I've seen this pop up elsewhere but didn't catch the bug until Barking Alien passed it on to me. So here goes...
1. Race (elf, dwarf, halfing) as a class? Yes or no?
Yes if I'm playing B/X D&D, otherwise no. I need a more nuanced class/race system.
2. Do Demi Humans have souls?
In my Lingusia campaign elves, halflings and gnomes are spirits of the weirding made flesh, so no. Dwarves are natural creatures of the earth and do have souls, however. In my Chirak setting elves have souls, as do all other living creatures. In my Enzada campaign absolutely, they have souls which must reincarnate like everyone else.
3. Ascending or Descending Armor Class?
I'll use whatever the system calls for, but I personally prefer ascending AC for play, while preferring the way descending AC tended to create a "hard cap" at -10 AC, insuring you didn't get the no-upper-limit problem of 3rd edition.
4. Demi-human level limits?
I personally never liked level limits and wouldn't use them anymore, unless I was going for a "purist" approach to whatever edition I was running just for novelty.
5. Should thief be a class?
Absolutely. Thief has been a class for what....38-39 years now? It was "not a class" for all of 2-3 years prior. So I think thief has earned it's place. Note however that "rogue" works fine for me with thief as a subtype/archetype/kit of such, too.
6. Do characters get non-weapon skills?
Absolutely. I invented my own skill system as a kid, to allow for more detail in characters. When you're 12 and don't know a lot about, say, siegecraft, engineering or astrology it's nice to know your character, who is supposed to be a master sieger, engineer or astrologer has something which helps regulate what he knows, even if you don't. That stance hasn't changed even as of today, and I have found that a robust skill system dramatically improves all the cool story bits, lore and esoterica that players might seek out because they have an incentive to do so.
7. Are magic users more powerful than fighters? (and, if yes, what level do they take the lead?)
This seems to be a mechanical, edition-dependent question. I guess the short answer is yeah, sure....because wizards in every edition eventually learn spells that let them do stuff beyond mortal fighter's abilities. That said....I do prefer editions in which the fighter is still relevant at high level (say, 10+) despite the fact that the wizard is flinging around prismatic spheres and disintegration rays. How else did Conan still manage to chop up half of the magic users in Hyboria, anyway? The best wizard spell is still solved with a swift jab to the midriff, after all!
8. Do you use alignment languages?
I thought this was a dumb concept poorly executed when I was ten years old, and I still do. I tried to run a purist 1E game with aliagnment languages last year and gave up after two sessions. It was just....no. When even Gary is writing in the book that it's not a very good working concept, and justifies it by pointing to other secret languages (that are only tangentially relatable to alignment languages) then you know it's a concept with issues. With one exception, however: alignment languages might work really well for a Planescape campaign.
9. XP for gold, or XP for objectives (thieves disarming traps, etc.)?
I discarded XP for gold when I was ten; it seemed like a weird "double reward" to me when I first read it....you get experience for acquiring treasure? It made no sense to me! Treasure was its own reward. So I worked out a mission bonus for completing levels/areas of the story, and only kept GP for a 1:1 earning for paladins and clerics who tithed their gold to the temple. I played 1E that way consistently, and when 2E came out I adopted it's methodology and never looked back.
10. Which is the best edition, ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E AD&D, 2E AD&D, 3E AD&D, 4E AD&D, Next?
Such a hard one for me to answer, as my default response is "The one that's D&D." But I guess if I go by "what I've played the most" I'd have to say 3E/Pathfinder because it's the edition that does the most of what I want, but 2E is the spiritual top dog for me....so I guess I'll say 2E (best overall) and 3E (best actual play). As for D&D Next...time will tell.
Bonus Question: Unified XP level tables or individual XP level tables for each class?
I have no preference here. A unified table is simpler, but it is less nuanced and requires a class system for the edition that insures that classes are balanced according to the same progression. If you have different progression tables then you can customize them to reflect different rates of advancement for each class, which means that you can treat the XP chart as one way to adjust for class balance and power. Ultimately it makes little difference for me either way.
Well that was fun....! It's an interesting list, built I think primarily for an OSR-heavy view of D&D, and the paradigms that are most commonly associated with the older editions, I suppose. I tend to treat each edition as it's own thing, and usually don't play favorites if I can help it. I'd rather toot the horn for things I like than gripe about things I don't, but it's generally interesting to see among these questions what are most acceptable. Ultimately the editions I play the most will be those which let me build and create the worlds I want with the greatest ease. My Chirak setting has minotaurs, animates, muskets and other strangeness, for example. My Lingusia setting was founded on 1E AD&D and B/X principles so it's the most malleable, while Enzada was built from the ground up to take advantage of what Pathfinder has to offer. Each of these worlds are easier to use in some editions than others. If I do get around to running a B/X game, for example, I'll probably write a new setting (or dial back Lingusia to an earlier age) to run it.