Root

A Root is a debuff that causes the affected unit to be unable to move or use any movement abilities. Rooted units are still able to use other abilities and basic attacks. Getting Rooted during a dash-based movement ability instantly ends it.

Blizzard on Root Abilites
According to Senior Game Designer MattVi, one thing Blizzard has to be careful about when using Root effects is that they clear the target's Move orders. This can feel super awkward as an enemy if you don't realize you were hit by a CC (why did my Hero stop moving?), and is the reason that the existing Root effects tend to be on very visually distinct, noticeable abilities. Blizzard sometimes opted to do a very large Slow instead of a Root in places where this felt problematic. To some extent the reduced effectiveness of Slows on high-mobility heroes (like Tracer) is important because they're dependent on their mobility, but you raise a good point that their relative value is affected by the prevalence of Slows.

Basic Abilities

 * Anduin's ability, "Chastise", roots the first enemy hero it hits for 1.25 seconds.
 * His level 4 talent, "Holy Reach", increases the root duration to 1.5 seconds.
 * Arthas' ability, "Howling Blast", roots enemies within the target area for 1.25 seconds.
 * Deckard's ability, "Scroll Of Sealing", roots enemies within its inner area of effect for 1.5 second.
 * Kel'Thuzad's ability, "Frost Nova", roots enemies within its inner area of effect for 1 second.
 * His level 1 talent, "Blighted Frost", increases the root duration to 1.5 seconds after gaining 30 "Blight".
 * Junkrat's ability, "Steel Trap", roots the first enemy hit for 2 seconds.
 * Malfurion's ability, "Entangling Roots", roots enemy heroes hit within the target area for 1.25 seconds.
 * His level 16 talent, "Tenacious Roots", increases the root duration to 1.5 seconds.
 * Thrall's ability, "Feral Spirit", roots enemy heroes hit by it for 1 second.
 * His level 16 talent, "Alpha Wolf", increases "Feral Spirit"'s root duration to 1.5 seconds.
 * Xul's ability, "Bone Prison", roots an enemy hero for 1.75 seconds after a 2 seconds delay.
 * His level 13 talent, "Amplify Damage", increases the root duration to 2 seconds.

Heroic Abilities

 * Jaina's level 10 heroic ability, "Ring of Frost", roots enemies hit by the initial blast for 3 seconds.
 * Kel'Thuzad's level 10 heroic ability, "Frost Blast", roots enemies in an area around the target for 1.5 seconds.
 * Mephisto's level 10 heroic ability, "Durance of Hate", roots enemy heroes hit by 2 seconds.

Talents

 * Arthas' level 16 talent, "Remorseless Winter", adds an additional effect to his "Frozen Tempest" ability, rooting for 1.25 seconds enemy heroes that remain within "Frozen Tempest" for 3 seconds.
 * Arthas' level 20 talent, "Absolute Zero", adds an additional effect to his "Summon Sindragosa" heroic ability, rooting enemy heroes hit for 2 seconds.
 * Cassia's level 20 talent, "Imprisoning Light", adds an additional effect to her "Valkyrie" heroic ability, creating a wave of light upon impaling a hero, rooting enemies hit for 3 seconds.
 * Cho's level 20 talent, "Favor of the Old Gods", adds an additional effect to his "Upheaval", rooting enemy heroes for 1.5 seconds.
 * Jaina's level 16 talent, "Numbing Blast", adds an additional effect to her "Cone of Cold" ability, rooting chilled targets for 1 second.
 * Rexxar's level 20 talent, "Kill Command", adds an additional effect to his "Unleash the Boars" heroic ability, rooting enemy heroes hit for 1.5 seconds.
 * Stukov's level 13 talent, "Virulent Reaction", adds an additional effect to his "Bio-Kill Switch" ability, rooting enemies affected by his "Weighted Pustule" in the area of effect of his "Lurking Arm" for 2 seconds.
 * Whitemane's level 13 talent, "Harsh Discipline", adds an additional effect to her "Searing Lash" ability, rooting enemy heroes hit for 1.5 seconds if they are affected by "Inquisition".
 * Zul'jin's level 13 talent, "Ensnare", roots the first enemy hero hit by it for 2 seconds.

Vehicle and Non-Heroic abilities

 * Both neutral and in-lane Bone Golems, Grave Golems, as well as neutral Garden Terrors, roots enemies in a circular area after a delay for 3 seconds.
 * A Frozen Punisher calls forth frigid ice shards, which explode after a short delay, dealing damage and rooting enemies in place for 1.5 seconds.

Counter Abilities

 * Becoming either Unstoppable, Invulnerable or entering Stasis cancels all root effects and makes the hero immune to roots for their duration.
 * Alexstrasza's trait, "Dragonqueen", reduces the duration of Roots by 50% when in the dragon form.
 * Ana's level 13 talent, "Purifying Darts", adds an additional effect to her "Healing Dart" ability, removing roots from allies hit by it.
 * Arthas' level 7 talent, "Icebound Fortitude", reduces the duration of roots by 75% for 3 seconds.
 * Brightwing's trait, "Soothing Mist", removes roots from nearby allied heroes.
 * Illidan's level 10 heroic ability, "Metamorphosis", removes roots affecting him.
 * Lúcio's level 13 talent, "Can't Stop, Won't Stop", adds an additional effect to his "Wall Ride" trait, making him immune to roots while its effects are active.
 * Lúcio's level 20 talent, "Synaesthesia Auditiva", adds an additional effect to his "Amp It Up" ability, removing roots from Lucio and nearby allies.
 * Lunara's level 13 talent, "Abolish Magic", removes roots from her and a target ally.
 * Malfurion's level 7 talent, "Nature's Cure", removes roots from allies affected by his "Regrowth" ability.
 * Mei's level 7 talent, "Backup Battery", removes roots affecting her.
 * Sonya's level 4 talent, "Hurricane", adds an additional effect to her "Whirlwind" ability, removing roots affecting her.
 * Sonya's level 10 heroic ability, "Wrath of the Berserker", reduces the duration of roots by 50% while in Berserker form.
 * Yrel's level 4 talent, "Hand of Freedom", removes Roots from the targeted allied Hero.
 * Zarya's level 16 talent, "Cleansing Shield", adds an additional effect to her "Shield Ally" ability, removing roots from the targeted ally.
 * Zul'jin's level 4 talent, "Voodoo Shuffle", removes roots affecting him.