I'm the exact opposite. When anyone says these things, it drives me nuts. It's even worse seeing them written out. Blech. I think that it was the way I was raised and the schools I attended. People often laugh at my "properness" during online communication (via Facebook, AIM, and email).
While those "words" irk me, I try to simply ignore them, because I know that language is ever-evolving, and I must be open. I've become less of a grammar Nazi as I've aged, and I even force myself to end sentences in prepositions sometimes, because that's what people have come to understand the easiest.
Sometimes, my writing (in prose) can be confusing (I blame Ne), so I am usually trying to write shorter and more succinct sentences. I also use parentheses often in casual writing (obviously), and I use quotations to denote irony.
I don't use many emoticons at all, or explanation points unless they're needed. It drives me nuts when people use both obsessively. They lose their effect. Are you really shouting and smiling during everything you say?
When I write creatively, it is often avant-garde poetry. The style captures my thought-process well. And I use a lot of imagery.
As far as your concern with ending sentences in prepositions, it is sometimes
correct to do so! It all depends on whether or not the preposition adds anything to the sentence. For example, the sentence, "This is where I'm going to" is improper because "This is where I'm going" does the same job without the preposition "to." However, "This is who I'm going with" is proper because "This is who I'm going" has no actual meaning, making "with" necessary.
I understand what you mean by people pointing out your "properness" over IM. A lot of people found it odd that I typed in full sentences with generally correct grammar (I'm one of those "gonna/gotta" people. :tongue

, but the people I talk to regularly have become used to it.
Unlike a lot of people, it seems, I have no problem separating my "academic" voice with my spoken voice. It probably has a lot to do with the fact that I tend to slur words together and be fairly lazy in general when I speak, and I cannot get away with that when I write papers because it must be polished and correct.
My academic papers can be confusing. None of the "pre-writing exercises" I was taught during high school and Comp I/II have ever improved my writing. They just make me feel limited and constrained in the structure of my paper, which I absolutely hate, but that has the disadvantage of leaving my rough draft
extremely scatterbrained, and that requires a lot of time for revising, editing, and proofreading of my paper--not to mention finding willing, competent people to revise my paper. I find the trade-off worth it because, in the long run, I save time by having every point I want to talk about down on paper in roughly the way I want to discuss it.
As for creative writing, I don't do a whole lot, but I usually like prose. Poems are fun, but that's only because I make them satirical or whimsical or silly. They do have a point, but I find the more "serious" poems to be too weighty and depressing, and that's what I reserve existentialist readings for!