Since the only items impossible to obtain in peaceful are mob drops (that are not obtained otherwise) and derivatives, here's a list of unobtainable items:
- Blaze rod
- Music discs (with the exception of music discs "13" and "cat")
- Ender pearls (even though you can obtain an eye of ender by trading)
- Spider eye
- Ghast tear
- Slimeballs
- Magma cream
- Nether star
- Wither skeleton skull
There are only a handful of direct derivatives:
- Brewing stand (requires Blaze Rod)
- This leads to no potions as well, especially those that use a Fermented Spider Eye, Ghast Tear, Blaze Powder, or Magma Cream
- Sticky Piston (requires Slimeball)
- Beacon (requires Nether Star)
- The Wither (requires 3 Wither Skeleton Skulls, but you're not going to spawn that on peaceful anyway)
- Fire Charge (requires Blaze Powder)
- Also limits available Fireworks
And here is the list of drops you can still obtain:
- Bone (can be found in temple and dungeon chests)
- Rotten flesh (before snapshot 12w22a could be traded with villagers but can also be found in temple and dungeon chests)
- Strings (obtained by: destroying cobwebs and tripwires and it has 53% chance of spawning in a dungeon chest)
- Arrows (can be crafted with other resources available in peaceful, can be found in temples and can be traded with villagers)
- Gunpowder (will be possible to loot in dungeon chests)
- Gold nuggets (can be obtained by placing a gold ingot into a crafting area)
- Dragon egg (reaching the end is possible by trading eyes of ender with villagers; apparently the ender dragon won't despawn on peaceful and the creation of a piston is still possible)
- Glowstone dust (can be obviously obtained in the nether and by trading with villagers)
It is also to note that a lot of items, that are possible to obtain, won't be renewable anymore, like:
- Bone (previously farmed by killing skeletons or wither skeletons)
- Coal (not charcoal, previously farmed by killing the wither skeleton)
- Gunpowder (previously farmed by killing creepers, ghasts or witches)
- Strings (previously farmed by killing spiders and cave spiders)
