He invented the original "rock star" game developer persona in the wake of his successes with Wolfenstein 3D, Doom and Quake, which more or less defined the FPS as one of the major genres of gaming.
Unfortunately, the rock star persona meant rock star ego, and Romero took it to the degree where he even had the rock star outfits and rock star hair.
Following Daikatana, he really hasn't been involved with anything significant, which is probably the main reason for the hate.
As long as a young developer keeps delivering, eccentric behavior, arrogance, quotable quips to the media, etc., even if just to stay controversial and topical, are generally tolerated, but once you stop delivering yet you insist on keeping the same wunderkind attitude, then you're pretty much just as ass at that point and aren't likely to get a lot of love.
If he delivered another major hit that could be turned into a franchise, I think most of the hate would probably be forgotten, but it does seem as though his most creative days are long behind him.
