Here's a list of simple changes that can be made to the D&D 3rd Edition game rules to bring them more in line with the tone of 1st Edition AD&D:

• Enforce suggested limitations on race/class options (DMG21).
• Use "Variant: 1st-Level Multiclass Characters" (DMG40).
• Use "Variant: Gaining Class Abilities" for training (DMG42).
• At 1st level, roll for hit points, but with a minimum of at least an average roll (half maximum), as per Table 2-23 (DMG42).
• At 1st level, don't allow wizards to choose their 1st-level spells; roll on a table specific to their school or specialty (giving one each offensive, defensive, and miscellaneous spell).
• Remove the "Massive Damage" rule (PH129), replacing it by making all falling/environmental damage cumulatively increasing (see www.superdan.net/environs.html).
• Treat the shield spell as granting a +7 armor bonus (not cover, as given on PH251. See www.superdan.net/adndfaq1.html for more explanation).
• Constitution checks are required for transformative magic (aging, petrification, polymorph others), at DC 7, or the subject dies. Any resurrection spell (raise dead, etc.) requires a similar check (at a +1 bonus) or the character is dead forever.
• Use individual weapon-versus-armor-class adjustments for attacks, directly from 1st Ed. rules. Alternatively, use the approximations to these adjustments, by weapon type, shown below: