So I just saw the threads regarding this subject, and BOY, let me tell you. Like so much association fallacy being slingshot to the extreme opposite.
This is not to say, that yes, Fel be tricky. I myself are not an expert, but given the information that we have, I have my own working theory that gets me through the day, keeps me warm at night and refrains me from raging about Fel in the forums. It might be total shlock.
Fel, above all, it's an energy source, as much as the light, the void and arcane are. Every one has it's special qualities, but Fel's most intrinsic one is that is created by destruction. "Burn Life to Create" . So what makes Fel magic fel it's its source. You obtain fel magic by sacrificing life.
Being a Warlock is all about choosing this, you can't go "I'm going to be a warlock, but no fel, like, that's gross" You might only sacrifice void and demon entities in your dark arts, but sacrifice is still the basis for your power. Fel is what powers you, a ready-at-hand vastly powerful force, and it is your willingness to deal in these magics (regardless of how often you do) what makes you a warlock.
All Warlocks use fel, but not all fel users are warlocks. Fel as a magic source can power and strengthen myriad creatures, but are only warlocks who use it as the source of their spellcasting; they can compliment their repertoire with other sources, but it is fel what powers them, it is fel the reason they left their mage and shaman paths.
Sacrificing life to power your spells is fell itself; which would actually explain why some blood elves' eyes turned green even when they were just eating up mana-wyrms.
So that's my theory, feel free to weigh in.