That depends. At work, certainly not. I can be quite kind and cordial since, ya know, it’s best for me to be that way.
With family and friends I’m harsher than here. I tell them frankly what I think of them. They wouldn’t be my family and friends if I didn’t.
So the SDMB is in between; I’m not well liked by the powers that be but I value the Cafe Society for food and tv/movies so I hold my tongue more often than not.
A little bit, if only because being rude online really doesn’t have the same consequences. 
I guess. With the exception of attempting to be scrupulously polite to everyone who seems polite to me, I’m rude to everyone I don’t know that well, or who ticks me off for some reason, unless I like them or they give off a friendly vibe. If they seem friendly, we become fast “friends” (of a sort), IRL, but absent that information, I treat people online like I treat every other boob on the street – some jackass trying to run me over with a bike or a car or get one over on me. Always suprised when your average person online is even remotely polite or decorous.
It seems to me that your policy of being rude to everyone you don’t know well, and polite to persons who seem polite to you, is self-defeating. That is, you increase the likelikhood of random persons being rude to you by this policy. It would be better to adopt a policy of default politeness & rude when necessary, or tit-for-tat.
I may have not expressed myself correctly or precisely – I merely prefer to not engage with people who seem standoff-ish, based on past experience of how well this actually turns out (for me). In fine, what I meant to say is that I don’t engage people who don’t put off a nice vibe. More correctly, I chat with anybody, at a bus stop, in a friendly way, if they have a question, or vice versa.
But people who come at me the opposite way, well, I either ignore them, walk away, or (in very circumstances) give them some RL snark.
Nope, that’s why I use my real name as my board name. Call me an “equal opportunity offender” 
Nope, although I read more than post. I don’t really see a reason to not be myself. However, I am more opened about what I post than in other public forums such as FB.
No. I’m actually nicer.
If anything I’m nicer here. I can be a bitch in real life (not that I can’t here, but it’s easier to ignore me here.)
I have found that I am polite when I speak or type in English. I am polite when I speak Japanese. I can be either polite or very rude in Chinese depending on the situation . . . much more rude than I could be in English. That was a surprise to me, that I could be rude in Chinese but not in English or Japanese.
My Korean language capabilities only allow me to ask basic questions. So if I am rude then it is accidental. But the Koreans seem appreciative that I can speak any Korean at all.
Please understand that I encounter hordes of foreign language speakers every day.
Much ruder in real life. But only with people I know.