I generally see too little formatting versus too much. No paragraph breaks, etc. The rules of writing syntax apply to forums as well, even if the culture of the forum (4Chan et al.) is crude or primitivist.
Bolding makes strong points stand out.
But, it's only good in small doses. The more you bold, the less the impact. Sometimes, you don't really need to use bold if you're using emphasis inline. Italics draw focus and lend weight to a key point. They're also useful for mixing in other languages, capisce?
It's been my experience that the folks who understand how to use the markdown of the forum in question are also the folks seen as veterans on the forum, which tends to carry some weight.