Originally Posted by
Shadow-cleave
The Alliance are not (at least, not with Arcane).
The Kal'dorei looked down upon Arcane magic (until recently), and those that use it are largely inexperienced (even the experienced ones, the Highbourne, are a lot weaker than the Sin'dorei - Will elaborate more on this later). In essence, they are "noobs," and prone to mistakes, as shown in the Horde's Azshara questline, wherein the player and a few Blood Elves make the Kal'dorei mages look extremely bad.
The Draenei are more focused on Light-based magics now than they were thousands of years ago, thanks to the Naaru. While they are exceptionally strong with those, there are not many examples of Draenei being very good magi (Like you said, those magi turned to Sargeras (as magi sometimes do, in a bid for power)).
The humans have a few examples of strong magi. However, many of these were Guardians, the result of a few mages pooling their power into one mage. Medivh and his mother are 2 of these examples. Jaina is another. Other than that, you will not hear about many.
The Horde, on the other hand, has both the Forsaken and the Sin'dorei. The Forsaken have whatever magi were killed and raised, both human and elven, allowing them to further increase their power by having more time to learn whatever magics they wanted.
And now, the Sin'dorei. They have many examples of powerful mages. Hell, their entire society is based upon magic. Elves are, in general, stronger than the other races when dealing with magical matters (Other than those few exceptions, such as Jaina, and even then, she is outclassed by a few). Furthermore, they are backed by the powerful Sunwell, recently restored, granting them further power. Elves have a longer lifespan and, while humans learn much in the limited time they have, elves can continue learning after humans perish.
Really, how can you possibly suggest that humans are more powerful with magic than a race who's entire society uses it every day, for even the most menial tasks?