It really needs no explanation. He's a terrible writer whose work is somewhere on the level of the average 14 year old boy on Fanfiction.net. But I'll crit you with a wall of text anyway.

Not only does he muddy the lore with his nonsense as someone else said (see: War of the Ancients and the timeline fuckery). He has no idea how to write legacy lore figures, and they end up with atrociously flat characterizations if they're not OOC entirely. Then there's his well-documented love of Sues. What makes him so terrible is that he never confines himself to one Sue. It's like he creates an entire web of Sue-ery to make his characters look cooler than established lore figures. The cardinal rule when you're writing fanfic is that original characters CANNOT upstage the canon characters. They're to serve a specific purpose. What makes a Sue a Sue is that they are always extremely wise and powerful special snowflakes and beloved by everyone important. Rhonin is a Sue not because he's a powerful mage; Rhonin is a Sue because he's a powerful mage who was mentored by the Most Beloved Consort of the QUEEN OF THE DRAGONS (himself a Sue), is married to an entirely made up third Windrunner sister (herself a Sue), had TWINS with her, and traveled through time to meet every major lore figure of the era, outdoing them in battle and generally being a smug, self-important ass. Hack apologists will often try to justify his Sues by arguing that because he's working for Blizzard, his "lore" characters are official and he has carte blanche to do with them as he pleases. I think that's a ridiculous cop out by people who wouldn't know good writing if it crit them on the ass. I can guarantee you that no Warcraft fan was buying War of the Ancients to read about the awesome time-traveling adventures of Rhonin. Hack took what was essentially a story about the Stormrage twins + Tyrande fighting the Burning Legion, and the struggles of the Dragonflights, and turned it into a Rhonin story. For Christ's sake, Rhonin was giving Illidan MFing Stormrage tips on how to use magic. Illidan, who was repeatedly stated to be an extremely gifted arcanist, even in this awful rendition of events.

And really, all of that is on top of the fact that he just plain cannot write. He regularly commits thesaurus abuse, was apparently asleep in his freshman composition course during the lecture on "show, don't tell", and is in dire need of an editor with a spine. I'm not kidding when I compare him to a badfic writer. It's like he goes through those "how not to write fanfiction" articles and uses them as a checklist, right down to using "orbs" as a synonym for eyes. Repeatedly. In the same paragraph.

I find that people who love him typically are the same kinds of folks who will read any sort of licensed novel about their favorite game, regardless of artistic merit. You run into the same mentality a lot in the various D&D fandoms, Forgotten Realms in particular. I haven't met anyone who actually reads good fantasy, or good fiction period, who likes Hack's Warcraft stuff. I have notoriously low expectations as far as licensed fiction, particularly licensed RPG fiction goes, and even I can't deal with his stuff. Not even the MSTie part of me that loves to make fun of trashy books. By comparison, I was actually able to get through Twilight to make fun of it. Take that as you will.