People say her power is substantial, but all we know is...
1) She's stronger than Mannoroth.
2) She's likely more powerful than Malfurion, though when they meet in Cataclysm she doesn't fight him, which would indicate that doing so would have been a considerable risk.
3) She's less powerful than Archimonde or Kil'jaeden.
4) She's most likely less powerful than Arthas or Deathwing.