Lines are quoted from the official notes; buff/nerf tags are auto-classified from the numbers and can be wrong. Covers the 14 most recent Dota 2 patches PatchMeter has fetched.
▼nerfBase Movement Speed decreased from 325 to 295
▲buffBase Damage increased by 4
▲buffDamage at level 1 increased from 38–42 to 42–46
✕removedRemoved Bear With Me and Bear Necessities facets
✕removedRemoved Gift Bearer innate ability. Its effect is now a part of Summon Spirit Bear ability
⇄adjustSummon Spirit BearMoved to an innate ability. Cannot be leveled up
⇄adjustSummon Spirit BearAbility is moved to the 4th ability slot
⇄adjustSummon Spirit BearSpirit Bear now counts as a melee hero for most spells
⇄adjustSummon Spirit BearSince the bear is now a hero, all unit-related changes moved to a separate Spirit Bear section below. This section is for the summon ability changes only
▲buffSummon Spirit BearCooldown decreased from 150/140/130/120s to 120s
▲buffEntangleLone Druid's Empowered Buff and enemy stack counter debuff are undispellable, Entangled debuff is dispellable
⇄adjustEntangleSpirit Bear's Entangling Claws reworked. Now it levels up with this ability and passively provides empowered attacks at all times
▲buffEntangleIncreases radius to 450 and hero stacks on cast to 5, instantly Entangling them. Also removes stack protection from Entangled enemies
✕removedSpirit LinkNo longer grants attack speed or shared armor
⇄adjustSpirit LinkNow also passively grants +10/20/30/40 movement speed to Lone Druid and +20/40/60/80 to his Spirit Bear
⇄adjustSpirit LinkLone Druid's attacks now heal the Spirit Bear by default
✕removedSpirit LinkNo longer upgraded with Aghanim's Scepter
▲buffTrue FormMana Cost decreased from 200 to 80
▼nerfTrue FormDuration decreased from 40s to 25s
▲buffTrue FormCooldown decreased from 100s to 60/55/50s
✕removedTrue FormNo longer grants Entangling Claws and Demolish to Lone Druid
⇄adjustTrue FormNow also provides 50/90/130 bonus attack damage
▲buffTrue FormBonus Armor increased from 8/10/12 to 10/15/20