One of our players this Sunday has played the living hell out of GURPS with us, and runs DF on his own, and has played D&D 5, too, I think.
But he's never played any D&D earlier than 3.5. Here is what I wrote to help him adjust a bit to AD&D (aside from some rules-specific writeups):
"AD&D vs. D&D3.5:
- much simpler combat. It's tactical only in the sense of using figures to determine location in specific set-piece battles.
- characters and monsters deal less damage (mostly) but have less HP (definitely).
- monster combat ability is purely based on their HD (hit dice), which also determine HP. Bigger monsters always have better "to hit" rolls.
- AC descends, and uses a table instead of ascending and using a "roll plus" system to determine a hit.
- Initiative is simpler, and individual alertness only reduces surprise.
- casters can't swap spells easily, or up-cast spells with a higher spell slot.
- the only skills in the system are thief skills and "to hit" rolls in combat.
- no feats or special abilities aside from some very specific class abilities, most of which won't figure in this game.
Offhand I can't think of anything else critical to know. It's just more lethal and less forgiving, based on my reading of D&D 3.5."
Anyone have any other helpful bullet points for a 3.5 player going to the confused and glorious mess that is AD&D? I'm extremely fluent in AD&D but 3.5 was one I only read without every playing. I know most of what I know from The Order of the Stick!