You can make the argument that Raven is one of the most underrated figures in professional wrestling history. If that seems like hyperbole, consider this: In the late 90's era of WCW, fans were conditioned to pledge their allegiances to either the NWO or WCW, to the point that even the pay-per-views were branded under those two factions, and for a time the most compelling character on the roster was someone associated with neither faction. Indeed, the character of Raven had such a cult of personality that WCW built a faction around him that existed entirely in the midcard yet factored prominently in WCW television.
Raven was never really going to work in the mainstream, at least not on a consistent basis. ECW was folk legend, but never meant for mass consumption, and Raven's tenures in WCW (the height of his mainstream appeal), WWE (where "dark, brooding character development" apparently meant "eyeliner"), and TNA (where they used ALL the eyeliner) all point to a talented wrestler that audiences could never get comfortable with as a major main event presence. While I'm not one to romanticize valiant "fan favorites" that never really break through to main event success (hopefully we all learned our lesson with Zack Ryder), Raven represents the rare instance in which a character appears to have genuinely been too effective, to have resonated too much with audiences, to cross over into major, main event appeal.
Raven made fans, particularly parents and young fans, uncomfortable. Heels like Hollywood Hogan or Kurt Angle may have behaved repulsively, but they didn't actually repulse viewers; they just tapped into the audience's anger. Raven was different. Raven tapped into something that audience's weren't even comfortable booing because they didn't want it acknowledged, let along writ large on prime time television. Raven tapped into the audience's fear.
The context of the times is important here. It's worth remembering that in a pre-Columbine world, the dread that every parent carries regarding their children and what they could possibly become was largely unspoken. The crippling responsibility, the knowledge that any wrong step by the father could be paid for by the son, it follows parents around. So when Raven showed up dressed not unlike so many of these students detailing a childhood filled with neglect and abuse of varying types and degrees, citing them as an explanation for his depraved worldview and actions, parents had a connection between childhood wrongs and adult maladjustment playing out right on their tv screens. Hell, the whole thing played out like a depressed, angry teenage rebellion taken to its logical (for professional wrestling, anyway) extreme. Watch Raven confront Terry Funk in the ECW locker room and you can see the tones and tropes from so many bitter confrontations between fathers and sons playing out toward violent ends.
On the one hand, it was absolutely compelling; on the other it was, at a certain point, supremely uncomfortable. Raven so convincingly conveyed the bitterness and angst of a ruined adolescence grown old that it pushed the viewer away by virtue of his unflinching reality.
His feud with Diamond Dallas Page in WCW remains, to many, his masterpiece, and is certainly the most far-reaching angle in which Raven participated. Having learned from the same mentor as DDP, Jake the Snake Roberts, Raven's early emergence in WCW, which revolved around vague occult rhetoric and a refusal to acknowledge the authority figures in place (poor J.J. Dillon had ZERO swag in those days), was focused on DDP's alleged refusal to help Raven achieve similar success to Page, despite having come up along the same path. His brooding, petulant, entitled cries of "WHAT ABOUT ME? WHAT ABOUT RAVEN?!?" perfectly contrasted against Page's hard working, self-made Amerian underdog ethos. Raven represented both the uprising of the unloved sibling and the horror of what that sibling's neglect reaps in adulthood.
Furthermore, the in-ring work between the two was superb. Both men were capable, versatile big men, able to use varying styles and degrees of technical proficiency, so rematches were never boring. The two men also had similarly built-up finishers. Everyone remembers how great the Diamond Cutter was, but it's worth noting just how over the Evenflow DDT was as well. Similarly able to appear out of anywhere, and devastating in terms of how it was sold, the Evenflow DDT was, inside the ring, the perfect complement to the "equal but opposites" vibe that the whole Page/Raven feud was designed to create.
The Flock continued to pick up steam as the feud progressed, cementing Raven as a demented, angry cult leader, and painting Page as the hero who could, by sheer individual will, turn away the darkness. Again, the fact that all of this storytelling was developed over a midcard title is unheard of today, but back then it set the standard for what a secondary title could represent; it was validation of how personal scores could be just as meaningful, if not more meaningful than major titles.
Raven made us squirm because unlike so many of wrestling's great villains, he was rooted in reality to a degree that we could see playing out in our daily lives. The generation of youth to which Raven spoke was one that was, unfortunately, keenly aware of the negative impact that abusive or neglectful parenting could have, and in Raven that darkness was given a voice. The feud with Page worked so well, really, because Raven was at heart similarly self-made, only where Page built himself out of pride, Raven built himself up from a foundation or rejection and anger.
Looking at how American youth have grown up since the mid-90's, it's hard to tell which perspective was more relevant to that generation. Raven was the compellingly dark fantasy of letting bitterness and anger and neglect wash over you until it hardens around your being to become armor, the sulking teenager locked in his room finally going over the edge and shutting everyone out taken to the extreme. As if that weren't uncomfortable enough, his Flock paid homage to just how enticing that sort of embracing of rejection can be, drawing people to Raven's message of freedom through anarchic revolution.
Raven was compelling because his story was happening in so many homes with so many teenagers and parents, making the nerves it connected with raw and sensitive. Other villains were great because they expounded on fantastic tropes of individual villainy throughout history; Raven was great because of how he managed to connect with a story whose ending was not yet known, because it was still playing out in reality as he was portraying it in the ring.