I'd like to pick up where everyone else here left off and weigh in.
I believe that the way we express ourselves on the Internet is important and can be misleading, since some people might intentionally act differently than they do in the offline world with an intention of achieving something, like drawing attention to themselves to feel popular, for example. Nothing wrong with that, really, as long as the person doing it isn't stepping on anyone else's toes in the process. You wanna make everyone think you're something you're not? Okay, whatever floats your boat is fine by me. Just keep your hands firmly on the helm and make sure your boat doesn't collide with mine.
I also firmly believe that a lot of misunderstandings that come from the lack of physical 'face to face' interaction can be avoided by simply using appropriate smilies to indicate your intentions and your emotional state. That's their purpose anyway. If a person never uses smilies and constantly talks in a certain manner that, given the lack of face to face interaction, can easily be misinterpreted as, let's say condescending, or downright hostile, it would be unfair and unreasonable from that person to expect anyone to think they're not like what they seem to be. Like
mrldii said: If it quacks like a duck, if it waddles like a duck - it's a duck. You don't wanna be perceived as something you're not? Great. Don't act like it. Act the way you want to be perceived. Simple, really.
As for spelling & grammar, it's something I make sure to do correctly. It's much easier to read & understand properly spelled posts than to fight your way through spelling and / or grammar mistakes. However, I also believe that being too uptight about the language and constantly insisting on doing everything 'by the book' can be detrimental, since most people don't have broomsticks shoved up their asses and are at least somewhat flexible in the way they talk, so, for example, when I see someone constantly using, so called, "big words" that even the most native English speakers don't understand without looking at the dictionary, I tend to cross that person off and simply not talk to him / her.
I also don't like it when someone uses non-default font, non-default font size and / or generally applies too many seemingly unnecessary formatting to their posts. In my eyes, it makes them look like an attention whores, but I certainly admit that could easily just be my personal bias.
