Taken from http://zioth.com/dragonsoul/stats:
According to the official documentation, Disables include to blind, stun, silence and petrify. Some examples are:
- Medusa's petrify
- Cosmic Elf's Silence
- Cutie's Stun
- Centaur's Blind
Some less clear cases include:
- Spirit Wolf's Mesmerize is not a disable (confirmed by a PerBlue employee).
- Faith Healer's Evangelize is a disable, since he has Improved Disables on his keystone.
- Understudy's food transformation is probably a disable (unconfirmed).
- Slows and and other debuffs (Frost Giant's Orange skill, Kaiju Green skill, etc) are not disables.
Putting Improved Disables runes on a hero who doesn't have disable skills has no effect.
Tenacity reduces the duration of disables, so it's valuable to any hero. The formula for Tenacity has not been disclosed, but experimenting shows that 450 Tenacity reduces the duration nearly to zero, and even a small amount of Tenacity helps.
How to figure out whether an effect is a disable:
If it's a multiple-target effect, you can go against a hero with that effect in expedition. If it ends on all of your team at the same time, it's not a disable, because Tenacity doesn't affect it. If it's a single-target effect, you'll have to count the seconds yourself, which is difficult in some cases, like Kraken's 0.5s stun. Credit for this trick goes to @Zenwhoa.