Telepathy (able to commune with his servants, such as Mannoroth and Archimonde, from the Nether)
Illusion (created a false vision of what he offered to the eredar in exchange for their obedience)
Invulnerability against mortal weapons, and limited immunity to immortal weapons (Broxigar's Axe of Cenarius was able to wound him, but not to any great effect, while the powers of the Demon Soul had no effect) as well as complete immunity to fire and fel magics.
Various magical abilities (lashed out with energy at Illidan via the portal being opened to summon him).
Able to look into the mortal realm through the eyes of a chosen servant (his "gift" to Illidan).
Ability to enter mortal bodies (remained within Aegwynn's womb for nearly a millennium) and take control of them (Medivh).
Body Flames (Sargeras’ flesh burns with demonic energy, scorching everything in his path. Sargeras’s body flames slowly increase the ambient temperature by up to 50 degrees in every direction for 1,000 miles. The effect on natural climates can be devastating.)
His molten armor can nearly burn your eyes.
The strength of 100 dragons is merely a fraction of Sargeras' strength; his strength nearly matches the power of a portal closing.
In comparison to Sargeras, Mannoroth and Archimonde were as fleas. This could be a reference to either power or height.
He wields the horrific broken sword, Gorribal.