I realize that "cool" is largely in the eye of the beholder, but at some point we all need external validation. Often a person has his circle of friends that (usually) agree on what's "cool." It is often the case that after a while the group's self-validation is not enough and the group wants external recognition of some form.
A good way to do that is to somehow display what you feel is "cool" to others outside the group and see if more desire to join the group. However, since there's a massive amount of information available from the 'net you can easily find people who share your interests, and if someone irritates you, you can move on to another group without looking back.
This is why I despise "|33+ $p3@k (leet speak)" and its variants. Frequently a newbie will mistakenly assume that it's "cool" and start to use it, not realizing that it turns people off because it's hard to read and just plain irritating, especially when you're trying to have a semi-intelligent conversation. It's probably not what led someone to talk to you, and it's almost certainly driving people away. In other words, it's not "cool."
I detest the use of mnemonics and the overuse of acronyms on the 'net. They're universally confusing for the uninitiated, and often confusing even to the "old salt" who's been online for years if he hasn't encountered the acronym before. (Remember, there are a huge number of them, and new ones appear every day.)
What I mean by mnemonics is the use of something like "How r u?" instead of "How are you?" This example is simple and almost readable, but try reading a group of sentences that substitutes "2" for "two", "to" for "too", "4" for "for", "r" for "are", "u" for "you", and so on. It's irritating.
It's also important to realize that it's much more difficult to come across as intelligent when you do this, and it frequently implies outright laziness. The most common excuse I see for doing this is not being very good at typing. But if you're a slow typist you'll improve quickly if you spend much time chatting online, and it's not a good idea to develop bad habits while doing so.
Acronyms are overused everywhere, but on the 'net it has become ridiculous. I am aware that some groups use them as a sort of rite of passage, where you don't belong in the group until you understand the group's acronyms. I can understand that, since it's not the focus of the group's interests. But I still don't agree with it.
Please get your system date and time configured correctly, and set up your email client properly. It's annoying to get email messages from the future, or distant past. As a matter of fact many people use a spam filter that scores email messages with obviously fictitious dates as likely spam. When I receive such a message it gets saved to a folder I rarely check, and when I do it's only a quick glance so I could miss your otherwise potentially legitimate message.
Proper use of punctuation greatly improves readability. Don't write several sentences without periods or commas; it's the written equivalent of the "Valley Girl" that rambles through several unrelated thoughts without pausing or taking a breath.
Don't abuse exclamation points ("!") periods (".") or question marks ("?"). Never hold down these keys until they start auto-repeating!
A related problem is the use of ALL CAPS. This is difficult to read and is almost universally considered shouting online. As stated in the previous paragraph, a single exclamation point is sufficient convey strong feelings.
(See my punctuation/spelling rant.)
In the same category of excessively repeated punctuation and all caps is the overuse of emoticons, or their use where it's inappropriate. The most frequently abused emoticon I see is the sideways wink. The commonly accepted use of this emoticon is to indicate you're making a comment that's intended to be humorous, but I see it tagged onto almost anything. For example, "Welcome back. ;)", "What are you up to? ;)" or even "LOL ;)" It's not appropriate in any of these three examples, even the last one because it's redundant. The tongue-poke ( :P :-P :b ;b and so on ) are almost misused as much as the wink, and just as annoying.
The appropriate use of emoticons can make it clear to your readers what the "emotional tone" is intended to be with what you're saying. For example, use a smile or a wink when a statement could otherwise be taken seriously when it isn't intended to be. Use a frown or a "tongue-poke" when you're indicating something serious or distasteful, and so on.
Related to inappropriate use of emoticons is the obnoxious habit of appending an "LOL" (which is wrong in and of itself, but so widely accepted that I ignore that fact—"LOL" means "Laugh Out Loud" but grammatically speaking it should be "laughs aloud" or "laughing aloud"), "haha" or something similar to every other sentence. I have seen people add one of these to literally everything they said before pressing enter.
Misuse of these devalues your use of what should be a written queue of your intended "emotional tone"; they become null information because the recipient of the message has to ignore them entirely.
Finally, the so-called "anime emoticons" are just plain irritating. They're much more difficult to figure out, and make sense only to those who are into anime. I enjoy anime, and I watch quite a bit of it when I have the chance, but I still hate these emoticons. (The most common one I see is: ^_^)
Websites that tell me to add the site to my bookmarks put me off the site. I feel it's insulting to your audience to tell them what actions to take, and it shows an insecurity in the quality of your content. Even "come back soon!" is questionable.
Likewise, websites that perpetually have an "under construction" notice tend to make me leave and look elsewhere. It is a fundamental fact of the World Wide Web that pages are always in a state of change, but if you can't "complete" your web page within a month or two, take it down.