The D&D module The Isle of Dread is for "Expert D&D", which is the Expert Set.
Are there any differences between the Expert Set rules and the AD&D that would prevent me running The Isle of Dread using AD&D? Could I even run The Isle of Dread with the AD&D rules without any modifications to the module?
Yes, you can run Isle of Dread as an AD&D module as-is, but there will be occasional hiccups. With only a few modifications (to the module), however, it can run as smooth as if it were originally written for AD&D.
I played through Isle of Dread in a 2e AD&D game as a player. I have asked the DM of that game for his opinion, as well as read through my own copy of the module. Here's what you would, in our opinion, have to watch out for:
The level/difficulty balance is roughly the same, or slightly easier. D&D classes were somewhat weaker than their AD&D equivalents, and that's without getting into special classes (which there shouldn't be much of in your game, since it's a low-level module). Depending on which optional AD&D rules you use, if any, that may change in either direction. Assuming it's a "vanilla" game, though, it ough to be comparable.
Keep in mind the standard issues of D&D -> AD&D reading, such as D&D's assumption that Elves can cast magic, or the lack of Good/Evil alignment (creatures and items will be described as Chaotic, Neutral, or Lawful). Generally (but not always) you can often infer Evil from Chaotic and Good from Lawful in these cases, but I would rely heavily on context.
No monster in this module is significantly different from its AD&D incarnation, power-wise. You may wish to rely on the stats blocks from your Monster Manuals instead. It's nice to have the ThAC0s and XP for each creature instead of having to calculate them from HD and special abilities. Also, you can get their updated alignments. This module is intended for low-level play, though, so it doesn't have any creatures that really play up the differences between the two editions (caveat: Dragons have always felt more challenging to me in AD&D, probably because of the expanded combat rules).
On that note, the island's exclusive/new monsters are not in the AD&D Monster Manuals, for the most part (on a quick flip-through I only found the Dimetrodon (MMII, p.52). They're probably somewhere in 2nd Ed's more extensive Compendiums, though. If not, be prepared to figure out their XP values.
Figure out how you're going to do poisons. There's quite a few poisonous critters and traps in this adventure, and none of them describe the effects of their poison. Go thematic (spider poison might paralize you, while a potion of poison might make you save-or-die at a slight bonus), or try and strike a balance with the party's resources in regards to weathering damage.
I would re-stat all spellcasters in the module to AD&D spell slots so that they're a match for PC spellcasters. For example, the 32. Witch doctor has 2 1st level spells and 2 2nd level spells. I would give him 3-3-1, as he is a 5th level cleric.
Not really a difference between D&D and AD&D, but remember that these modules were meant to kill foolhardly and unlucky PCs through both attrition and unbalanced encounters. If your players don't learn to respect the wandering monsters, they can easily die. This is a feature of the module, which is about teaching you how to survive wilderness adventures.
Note that the module has some mistakes in it, like giving multiple types of equipment to a group of NPCs but not accounting for AC differences (see 7. Pirate Lair). They're few and far in-between, but players might notice them.
