Here's the interesting bit - it seems to me that 1e had a much steeper learning curve than 3e or 4e. The later editions have unified mechanics with simply stated systems. 1e had a hodgepodge of systems with unrelated designs and complex resolutions that required reference to charts to get things done.
We are talking about "balance" here. It seems to me that the numeric balance in 3e and 4e is far more transparent than that in 1e, which should make learning what the DM has to add to keep things running smoothly easier.
I think the real difference is that there's more details in 3 and 4e than in earlier games with skills, feats, etc. Earlier editions didn't have that, though there were proficiencies in 2e, and IIRC the Basic game had a skill system somewhere. But where in 3e had standardized the d20 roll where high rolls were good and low rolls weren't, earlier editions had situations where high rolls were good for somethings and bad for others.
Though another problem is when 1e was rolled out, the game was set up into D&D and AD&D. New players were to start with D&D and move up into AD&D if they wanted to, and the systems were more compatible. But then when D&D branched out into BECMI, two seperate rule sets developed, and during the 2e days, D&D was taken off the market, leaving only the more complex AD&D. That was the situation when I started playing. I didn't have too many problems learning how to DM, but I can see how some new players at that time (a period of about 5-6 years) would have struggled. When 3e was released, it was an extention of the 2e rules, but some of the design decisions must have been influenced by the fact that it was the only D&D game on the market, and needs to be accessible to new players. The same considerations were made with 4e as well.
I think there's a separate learning curve for DMs and players. In TSR editions, players have a less steep learning curve. They could sit down, roll chars and start playing while relying on the DM to know and adjudacate the rules. But because of the heavier reliance on the DM's understanding of the system for the game to function, the learning curve for the DM is steeper.
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Sure, the DM has more power and more leeway to simply make things up. But learning how to wield his power properly is the big hurdle. Maybe the learning curve for DMs of old editions is less rule mastery and more game management.
A poor or novice 3e and 4e DM can always fall back on solid (more or less) rules and run a decent if unspectacular game. A poor DM running old editions can be left floundering and do a really crappy game.
Yeah, this was the point I was trying to make. The DM doesn't just have to learn the rules, he has to learn how to run the game. Running the game is something that needs to be learned by experience. The DMG gives some advice, but naturally has never been big enough in any edition to cover every situation a DM is going to face. That's were Dragon came in in the old days, and now we have internet forums where DMs can get advice, learn refereeing tips and so on. Even then, sometimes a DM just has to run the game to learn what needs to be done. The old Basic game kept things simpler for the DM as well. However when it got to the point during 2e when AD&D was the only version of the game, new DMs had nothing but a DMG that offered a lot of vague advice and encouraged them to do what they felt was best. As a relatively new DM, that certainly didn't help me much. I was never sure for example what magic items weren't too powerful, how much treasure to give out, how powerful a monster was compared to the party, etc. The 3e rules had some guidelines in place, although imperfect in spots, but it did give me a better idea of what to use.
Now I'm just talking about inexperienced DMs here. When it's a case of a crappy DM, it's going to be a pretty crappy game no matter what edition is used. The only real advantage to post-AD&D rules in this situation is that the DM may not wreck the game as spectacularly as he might have done in the past. That's not a guarantee though, rather that crappy DMing just has a more uniform result.