Yes, I think that legitimate criticisms can be cloaked in terms offensive enough to constitute a personal attack. However, this is an extremely subjective thing and reasonable people can dramatically differ on what constitutes "arrogance", "condescention", etc. I also think that game designers should recognize that criticism does come with the turf to some degree.
And agreed that a defense of a game you like isn't necessarily an attack on critics of that game -- until you do things like insinuate that they are just being irrational, bear an unreasonable grudge against the game, are jerks, etc. These things are all tactics used quite often by the "pro" side of the debate. These tactics become especially offensive (IMHO) when they are offered in lieu of a response to the criticisms.
Of course, it takes two to tango. Sometimes, a criticism is framed in terms that do not allow a response. "I just don't like it", for instance. In that case, it might help to recognize that this may well be an accurate statement, made in reasonably good faith. So instead of responding that the person is being irrational or is stupid for being unable to explicitely identify what he dislikes, you might try something like "well, I can't really respond to that because we are discussing a matter of personal taste and you've given me nothing to go on. Perhaps you could identify what mechanics you dislike?"
There is only one person that I can recall ever responding to in the way you have said (regarding unreasonable grudges)...and I stand by what I said. I believe this person's subsequent posts bear out the truth of my statements. I do not recall EVER accusing anyone of being stupid though.
And if someone basically says to me that if I don't agree with them then I obviously do not know what Traveller is all about...that is condescending and arrogant and I don't know any reasonable person who would disagree with that.
Allen