Well, there are worse sins than writing self-insert fantasies. The problem with Terry Long is mostly, I think, that Wolfman made him aggressively "normal." He's very creepy at first, but once he settles in, that tones down and he's more just... an ordinary guy. He's a mediocre college professor of no real note, he's a divorcee, his friends are coarse, wisecracking schlubs, he struggles with writer's block, there's nothing special or destined about him, he's JUST LIKE YOU AND ME! Which... I guess sounds like kind of a neat idea for the first ten seconds - "an extraordinary woman with an ordinary guy!" and all - but it just doesn't work. Ordinary people don't belong in superhero comics. I mean, you look at Clark/Lois, and the truth is, Lois is every bit as extraordinary as Clark is. She doesn't have his powers, but her courage, her persistence, her talent as a reporter and passion to use that talent for justice, they make her special. It's always been clear why Clark loves Lois.
But Terry..? Terry is so determinedly, persistently, meticulously not particularly special or interesting in any way that the idea that he fits in with the Titans, that he's worthy of Donna, that his life and their lives should overlap at all, much less intertwine, makes no sense. The way these extraordinary characters respond to all these ordinary chumps doesn't feel organic or rational. Every scene begs the question "why is this guy here?" And the only explanation is that "well, Wolfman wants him to be." Which just... never, ever goes over well with the fanbase. Doesn't mean that Wolfman himself is necessarily a creep, just means that his writing experiment failed in a really big way.