Nothing frustrates me more than when somebody tries telling me that something I've been doing for years is completely wrong. I was showing a friend one of my sprites, and she told me that I was doing it wrong because Pokemon sprites only use "2 main color and 3 background color pallets each", because the Pokemon sprites use "3 colors 97% of the time".
In my opinion the original sprites lack SO much detail that I've just subconsciously added more and more as I sprite, and, not trying to sound narcissistic here, but I like my sprites. I understand that a lot spriters have their own style, and that's okay, you're comfortable with what you like, and I'm comfortable with what I like, BUT DO NOT JUST TELL ME THAT IT LOOKS WRONG. A suggestion like "I like it, but maybe you should X instead of Y" is more polite and still critical, which I can accept or deny anyway, but TELLING me that my way is WRONG is an automatic way to make me ignore you completely, as pompous as that sounds.
*breathes in heavily* I'm sorry~ I was mad and had nowhere to vent~ Some people just have no idea how to critique properly without hurting feelings.