There's a variety of Ad Hominem attack that focuses on association and motivation, rather than the argument itself. For example: "That's sounds suspiciously like the sort of thing a closet OSR advocate would say." Or, "It is clear by now that you are just a 4e fanbot."
Put that way, these are clearly bad arguments, but the reason why may not be obvious. It's not just that the accusation might be obnoxious. It's that, even if the accusation were true, it has no bearing on the argument being made*.
I could be the Grand Kleagle, or a fifth-columnist, or eat kittens, or think Casablanca was a bad movie. Even if true, those qualities will not invalidate my argument. If my argument is invalid, you should be able to show it from the argument itself.
*The exception might be an appeal to your own contested authority. A scientist who makes a bold claim has been caught falsifying data before. A politician claims the mandate of the people, when he has been involved in electoral irregularities in the past. Even then, that may only be reason to give the claim greater scrutiny, not to invalidate it.