1. While I agree, common sense is a lot rarer than you believe it to be and a lot of people have difficulty properly portraying a character too different from their IRL self. You'll notice that if you RP with 5 characters from a person, you generally know what to expect when you talk to the actual individual. This doesn't apply to all, a lot of people experiment with different personality types, but it applies often enough to be a generalization.

2. Well, consistency issues are proof that you cannot play your character properly (regardless of gender, personality, etc.) If I'm a guy, I role-play a character named Mike who's shy and introverted, but start a conversation with the prettiest girl in the setting with natural confidence and a matching smile, I'm not giving Mike justice. In that context, I'm bad at playing Mike. If you play Mike and - while there is no prior mention of him being a cross-dresser - you have him go to the bathroom and slide his panties back on before straightening his skirt, you'd be equally bad at playing Mike. Of course, these are extreme examples, but those are two wrong ways of portraying that same character. They're both equally wrong due to consistency issues, but the consistency in question is completely different. One of them is because someone is poor at portraying a shy individual, one is because someone is poor at portraying a male.