Basic grammar and spelling is essential to me. Anyone who does not practice it baffles me. I don't understand the point of being lazy about it. You don't need to spell everything super correctly and a few typos are here and there are all right. But at least try to capitalize, use semi-decent punctuation, and periods. Why this has even become a thing on the internet will be something I'll never understand. It just makes it more difficult for me to understand what others are saying, and more difficult to read their posts. Using all lowercase just makes everything slog together into one giant jumble and I typically skip over it. What I say is if you can't make enough of an effort to make what you're saying follow very basic laws of English we learn early on in schooling, then I can't be bothered to read it, and will likely think of you as a lazy person/idiot who is not worth my time. (Unless you have a legitimate reason, such as English not being your first language. Most people do not have that reason.) Sounds a bit harsh and snooty, but that is how I feel. I pretty much read those who do not format their posts like that with slurring drunken voices in my mind. It's the text equivalent of speaking like that. You could be the smartest person in the world, but if that's how you chose to display yourself, then that is how my mind will judge. It's difficult not to if you deliberately chose to lower your standards.
Of course, I write semi-similarly to how I speak, but they're pretty far off from each other. Writing is how I've been expressing myself for years, and it affords me the time and thought my slow processor of a brain needs. Because of that, I'm far more eloquent and coherent here. I consider my written word to be my truest voice, much more sincere than the way I speak. What you are reading here is more me than hearing my voice. The way I speak is kind of messy and sputtery, often involving random pauses and mispronounciations of words because my brain cannot keep up with my mouth.
I try to avoid emoticons as much as possible, though I use them sometimes so people won't think of me as an intimidating text robot. But I feel they ruin the purity of my intent with insincerity, as I'd rather use them to portray REAL emotions I feel. If I'm smiling in real life, then I might use a smiley face.
Also, I don't say "lol" or any of that jazz. I feel it adds more to the insecurity of things, and is so overused that it means nothing nowadays. I don't like any unnecessary additions to sentences that ruin the sincerity of my words. What I want to do is come across as authentic as possible.
I tend to be semi long-winded in my posts (not quite to essay levels though, because I tend to fizzle out before them), but I will only post if I find it something worthy of being long-winded about. A forum for me is for long amounts of speech, whereas IM is for shorter messages. Anything I have less than three sentences to say about isn't worth posting about.