It's perfectly fine to swear or use slang, since everyone is unique..as much as massive amounts of conflict often occurs when two unique people with alternating opinions meet happens, but still. I generally don't mind swearing so long as it's not done in an abusive matter, and not constant. Example, you'll see me swear every so often but you won't see me conflagrate my post by swearing multiple times in every sentence...nor should you (hopefully) ever see it aimed at anyone else.
Slang is..meh. It's not slang that bothers me, it's accents. If someone has an annoying accent, I'm going to want them to shut up, regardless of the slang they use. If it's a pleasing accent, slang is generally okay. Slang when it's being written down is annoying though, since I always saw slang as a kind of "in build speech filter" of sorts you get hammered into your cranium when you grow up...there's nothing forcing you from typing it down, but there is a mental block making you say it in reality.
Perhaps, anyway. Regardless.
Edited in sudden thought: I actually hate people more who attempt to fake-swear than people who actual swear. There's a guy I watch from time to time on youtube called Necroscope86, and he has a massive habit of avoiding swearing by replacing words like **** (assuming it's not censored) with the word funk, and so on. It's just irritating and I feel like it's defeating the object, since you are basically cursing, only you've replaced an expletive with something stupid.
I don't mind the typical FLIPPIN HECK or HOLY CRAP as opposed to the stronger alternatives, but when it's just a word that has a similar written sense but otherwise is nothing to do with the original it...just grates on me. Dunno why.