I am not opposed to race as class mechanics. Hell, I started with Moldvay in the summer of 1980. On the other hand, I also enjoy the variation of race separated from class that came with AD&D and I recently discovered Swords & Wizardry: Complete and that is going to be a big influence on my fantasy gaming from here on out. Short and sweet, no.
(2). Do demi-humans have souls?
Of course. Every sentient being has a soul.
(3). Ascending or descending armor class?
BOTH - Swords & Wizardry.
(4). Demi-human level limits?
I will know when someone gets to that point. I am thinking they will continue to advance but suffer some sort of higher XP requirement or something.
(5). Should thief be a class?
Yes. A thief is a specialist but not the only character that can sneak, hide, etc; but they are more proficient at those types of activities.
(6). Do characters get non-weapon skills?
Yes, they get whatever is appropriate to their race, class, and background. I try to shy away from a long list of ever expanding skills.
(7). Are magic-users more powerful than fighters (and, if yes, what level do they take the lead)?Certainly. Just think of some of the spells coming down the line for magic-users.
(8). Do you use alignment languages?
No. I could fly with stuff like a Demonic or Angelic tongue but NEVER one based purely on alignment alone.
(9). XP for gold, or XP for objectives (thieves disarming traps, etc...)?
I am leaning towards no XP for gold.
(10). Which is the best edition; ODD, Holmes, Moldvay, Mentzer, Rules Cyclopedia, 1E ADD, 2E ADD, 3E ADD, 4E ADD, Next ?
The truth is that whatever edition you are playing and enjoying is the best edition. For me, I have to say that Moldvay is the best - not because of rules or anything else similar - because it was there at my introduction to the hobby. Yes, I have a soft spot for Moldvay, Holmes, and AD&D because of the late nights my buddies and I spent rolling dice and exploring dungeons until we had to succumb to slumber.
Bonus Question: Unified XP level tables or individual XP level tables for each class?
Individual tables allow the "weaker" characters to advance faster than the "stronger" characters and serves as a better balancing factor than a unified XP table.