To amplify something from a comment Tavis made:
it’s taken a lot of on-the-job training for me to understand the old texts for what they are, rather than the assumptions I bring to them. Without a lot of intentionally trying to stick just to what guys like Gygax, Arneson, Bledsaw, and Jaquays actually wrote, the way I would have winged it would (I think) have been very unsatisfying & have missed the essential concepts of old school as a method, rather than an ideology.
As a player I have a feel of danger, pace, and range of possibility based on memorized portions of the Players Handbook, DMG, and Monster Manual. I struggle with my AD&D-based assumptions in two ways.
First, it is hard not to apply out-of-game knowledge in a way that violates what our characters would know . This is role-playing 101; it is always hard to be mindful of what you should and should not “know,” especially in the heat of the moment.
Second, related to what Tavis says above, AD&D and OD&D differ in a lot of minute details which contribute to the old-school “method.” Those details accumulate and create a wholly different environment, where assumptions based on the AD&D ruleset are not only wrong, but can hobble the OD&D feel.
One fine example is the different way magic swords are treated. AD&D swords are most often undistinguished artifacts, nice to have but not necessarily game-changing. OD&D magic swords are all unique, each having at a minimum an alignment and intelligence, and 60% having at least one special power, making them a watershed magic item for any party.
Another example is spell descriptions. The OD&D spells as written are generally less restricted and more open to interpretation. As a result they can be situationally more or less powerful and I think contribute towards a looser game, more open to possibility. Some examples:
- The fifth-level cleric spell Insect Plague in AD&D causes 1 hit point damage per round, remains fixed where it was cast, and lasts one turn per caster level. The OD&D version does no damage and only works above ground, but can be directed anywhere the cleric wishes, and lasts a day.
- Hold person in OD&D works as a powerful charm person spell, allowing the caster to order about and make use of the 1-4 creatures affected. In AD&D, hold person simply freezes victims in place, much less useful.
- Magic-missile in AD&D automatically hits. In OD&D, there is no magic-missile spell until introduced in Supplement I, where the text describes a “conjured missile equivalent to a magic arrow, and it does full damage to any creature it strikes.” Given no other context but the OD&D books and Greyhawk, it is reasonable to rule that the M-U has to make a to-hit roll for a magic-missile, drastically changing the nature of the spell.
The accumulative effect of all the minute changes in the game seems to be an expanded range of what can happen – a given spell, item, or encounter can more easily cause a dramatic change in the fortunes of the party. This can be the case in AD&D too, of course, but the tighter descriptions and increased detail make it harder to play the looser, pulpier, sword and sorcery game that OD&D arguably was, and the accidental application of AD&D rules can undermine the more free-wheeling play of OD&D.
 A recent example from our white box game: upon learning that the nameless patriarch is piling wooden sticks by a door, we immediately realize he is planning to cast sticks to snakes. With the possible exception of the one cleric in the party, would we have known that?
 And wands! In OD&D many of the detection and utility wands do not have charges, they may be used perpetually! For someone used to jealously guarding the charges in, say, a Wand of Enemy Detection, this can change the nature of how a dungeon is traversed.
 Which is exactly what Dr. Holmes did, specifically calling on pg. 15 to “Roll the missile fire like a long bow arrow (Missile Fire Table).” It may be the original (but unwritten) intent was for the magic-missile to be unerring, since it was so specified (corrected?) in Moldvay on pg. B16, “It will automatically hit any visible target,” and remained so in Gary’s Players Handbook version. Of course it could also have been an evolution of the spell based on play – only someone who played with Gary in those early games would know for sure how he treated the spell originally. If YOU did, please weigh in. The Mule wants answers!