Only about 5-10% is specifically multiplayer, the majority of content is designed to fit a single player experience (or PVE). Just a brain dump of a few things from the mod that are single player focused (
this is by no means everything just what springs to mind):
Poppets: protect yourself from various ways to die / take damage, auto repair tools and armor once
Dreamweavers: long time speed, mining and hunger removal buffs
Crystal Ball: Grant yourself interesting predictions of the future
Brews: help with bridging, ascending, jumping down holes, digging, farming animals and leaves, escaping mobs and landscaping, summon minions
Rites: Find diamonds with a helpful pig, bring lava to the surface, landscaping, grow forests, change biomes, area teleport across dimensions (players, mobs (get them mooshrooms home!) and items), underwater ice domes, make rain or night, trade items with demons, recover despawned items after death, create villagers from zombies
Wand/infusion magic: teleport like an enderman, take control of mobs and steal their powers, become invisible at will, dig holes, disarm pigmen, slow and pushback mobs, start fires, create water, Burn Souls, Pull Items, Stun, create light, heal, auto smelt, ore double.
Base Defense: Traps, Brambles (some teleport mobs), Leaping Lilies, Sentinal Scarecrows, Disorientation Scarecrows (mobs attack each other), new wood types
Gear: Charms to Trade with Animals, Gear to Protect from undead and creepers, Bark Belt (unique (I think) armor mechanism), Biting Belt & Parasytic Lice (auto potion injection). Crossbow Pistol with different ammo types. Fast flying/exploration with a Broom! Multiple hats, robes, shoes, belts, etc.
Bosses: Huntsman (drops Epic Spear), Baba Yaga (drops Epic Hat), Lord of Torment (drops Epic Spell), Death (drops Epic Full Gear Set and a summonable horse).
Minions: Classic Undead (you can give them weapons & armor too), Spectres & Banshees, Coven Witches, Imps & an animal Familiar.
Check out the
FTB wiki for all the features.