The two I see invoked a lot are:
1) What one player / GM considered fun isn't necessarily fun for other people.
2) Just because it is a different style of play doesn't make it wrong.
To me -- these are obvious. But they don't really add anything to the debate, and could easily miss the point of one of the other people involved. The person responding to one of these could easily just delete the line, continue reading what else the person replying had to say, and answer that -- the two points listed above are a non-issue.
Me: I like games where unexpected failure and death are an option. I like the idea that the PCs can fail, and fail spectacularly -- but it is right up there with the chance that the PCs, through cleverness, thinking ahead, and luck, could win spectacularly too. I find these kind of games fun.
Reply: <insert #1 here>
Which, of course, completely negates any intelligent discussion, since #1 is kind of obvious. It completely misses the idea of discussing what different players and game masters like, and what they'd like to see in a game, or reminiscing about spectacular wins / fails in people's personal campaigns, and so forth. And if #1 is stated, followed by such examples? There wasn't any point in even mentioning #1 now, was there? The OP would probably simply say 'well, yeah, of course', ignore it, and continue the discussion based on your other points.
Anything else do you think could be added to the list?
(Edit: And, by the by, thank you for all the replies which followed the spirit of the post. :) A lot of very good points were raised.)