Perhaps my experience has been too limited, but it seems to me that the majority of players I've encountered pretty much agree unanimously that, when Arthas died, the game felt like it was "over".
Obviously, Arthas was hardly the most important character in the Lore, and there is a great deal of compelling story just waiting to be told. Still, for some reason, people seemed to really connect with him, in a way that hasn't really been replicated since. And honestly, it's hard to disregard the timing on when WoW started to see a decline in its subscription-base. It would be foolish to pin all of it on Arthas's death, of course, but I think it's probably still at least somewhat relevant. After all; the Warcraft series had been building Arthas up for nearly ten years; killing him was bound to leave a void.
Plus, let's just be real; Arthas is undoubtedly the single most iconic Blizzard character around now -- hell, he's one of the most recognizable characters in Gaming, or possibly any other media by now! Show a picture of the Lich King to anybody, and I would bet money they know who he is, or at least recognize him to some degree. Even now, YEARS after his death, he's the poster-child for Blizzard's Heroes of the Storm. Frankly, his popularity stretches far beyond the Warcraft series itself.
In short; dude is pretty popular.
Honestly, I think the franchise took a major hit when he was killed, and personally, I think they should consider bringing him back in the future. Obviously, that's a pretty massive feat to undertake, as I think the idea of reviving him is something that, even for those who might agree with the core concept, would be a hard sell. Because Illidan is half-demon, his reappearance is pretty much guaranteed (demons never die, only get sent to the Nether), whereas Arthas' fate is far more vague. We know in Sylvanas' short-story (which admittedly was fan-written, and thus isn't hard canon) that Arthas was in some kind of dark voice being tortured. We also know that nobody is quite sure what happened to his body, or the shards of Frostmourne.
Even as someone who supports the idea, Arthas' return could easily turn into a cringe-worthy affair, if not handled with great care. I would probably stick Christie Golden on the case, as she seems to be the best writer that Blizzard has worked with yet, but she's clearly worked with the character before. Again, I'm not really sure how they could even revive him without it seeming forced, though.
Still, I can't help but feel like Warcraft has been suffering a major identity-crisis since Arthas died, and frankly, I just think they *need* to bring him back, in some capacity. Either that, or man the hell up and have us face-off with the heart of the Burning Legion, on Argus. Stop back-peddling and having us face weaker threats and villains.