Balesir
Adventurer
Interesting topic!
Like several others, I really don't have any specific "pet hates" that will turn me off an RPG at first glance; maybe that's part of the reason I own so many of the darned things!
Once I have begun to understand what the game is "about", however, there are some criteria that affect whether or not I want to actually play it. Generally, they boil down to this:
"It is imperative that all of the players, including the GM (if there is one), get to make interesting, meaningful choices and decisions that contribute in a compatible way to the progress of play."
Things like "players get control over all aspects of their own character" can fit, here - but only if that is compatible with the overall aim of game play (as designed). For D&D I very much want players to control their own character completely - but for HârnMaster it's unnecessary and for Universalis it makes no sense whatsoever.
Games that say one thing and deliver another drop out of this as a 'pet hate' - but it's often not clear that this is the case until I get into actually studying the game. Games that promise "you can play a craft guildsman, a skald or a lady-in-waiting" and then proceed with pages of weapon and armour tables and a book of combat spells just make me sigh. So, I can play "a craft guildsman, a skald or a lady-in-waiting" who quits to become a badass mercenary? If I create "a craft guildsman, a skald or a lady-in-waiting" I want to play them as such; if I am going to play a badass mercenary, cut the c**p and let me create a character who is a badass mercenary, for goodness' sake!
Like several others, I really don't have any specific "pet hates" that will turn me off an RPG at first glance; maybe that's part of the reason I own so many of the darned things!

Once I have begun to understand what the game is "about", however, there are some criteria that affect whether or not I want to actually play it. Generally, they boil down to this:
"It is imperative that all of the players, including the GM (if there is one), get to make interesting, meaningful choices and decisions that contribute in a compatible way to the progress of play."
Things like "players get control over all aspects of their own character" can fit, here - but only if that is compatible with the overall aim of game play (as designed). For D&D I very much want players to control their own character completely - but for HârnMaster it's unnecessary and for Universalis it makes no sense whatsoever.
Games that say one thing and deliver another drop out of this as a 'pet hate' - but it's often not clear that this is the case until I get into actually studying the game. Games that promise "you can play a craft guildsman, a skald or a lady-in-waiting" and then proceed with pages of weapon and armour tables and a book of combat spells just make me sigh. So, I can play "a craft guildsman, a skald or a lady-in-waiting" who quits to become a badass mercenary? If I create "a craft guildsman, a skald or a lady-in-waiting" I want to play them as such; if I am going to play a badass mercenary, cut the c**p and let me create a character who is a badass mercenary, for goodness' sake!