1) Of course every person should do their best not to insult anybody, especially not to offend individuals who already feel marginalized. Of course we should do our best not to say things that are transphobic, homophobic, sexist, racist, disrespectful to people with disabilities, etc.
2) In general, a good rule is to be empathic. To do our best to understand how another person might feel, how we might feel if we were in their shoes - and to act accordingly.
3) I personally prefer when people say that we need to "be thoughtful" or "be empathic." "Be politically correct" sounds too vague to me, especially because political correctness is understood quite differently by different individuals. Sometimes some forms of political correctness do go overboard, and people do read too much into somebody's words. This is my personal opinion, of course.
4) It saddens me that when somebody says that political correctness can be tricky, plenty of people already get offended. Political correctness should be something we discuss, not a generalized rule: "Either your are politically correct or you are a jerk." There are a lot of gray areas here. They should be used to start a dialogue and not to get mad at each other.
5) They say, the rule is that you cannot decide whether you offended somebody or not. The offended side decides for you. If somebody says they are offended, then you did something wrong. Well, yes and no. Of course, if somebody says they are insulted, you should really think about it and see what you could have done to offend them. On the other hand, I know people who get insulted all the time just because it's their thing. I can think about abusive relationships where one partner always complains that they are hurt and offended and uses the feeling of guilt this generates to control the other person. And what about some religious people who get insulted because same-sex couples can get married now? What about people who get insulted when they see a same-sex couple holding hands in the street? What about people who got offended by Beyonce's political statement? You cannot simply say: "If you insulted somebody by doing <insert something here>, don't ever do this again." It's just not that simple.
All in all, while I am certainly all for not insulting people (especially underprivileged individuals), I just want a bit more nuance with the PC debate. Let's not make generalizations, like: "Everybody who questions PC is against it." Let's not make rules that cannot be discussed, like "Always be politically correct, or else..." Some people say offensive things because they do not know better, not because they are horrible insensitive monsters. If you immediately get mad at them and start preaching, you might close all opportunities for productive discussion. Let's do our best to respect each other and be empathic, and let's keep talking about the complexities of human communication.
And here is a video that I find really relevant to my point.