IF YOU TYPE ALL IN CAPS, PEOPLE WILL TAKE YOU SERIOUSLY!!!!!!11
Silliness aside, there are general problems in pure-text communication. Sarcasm can be hard to detect unless it is made obvious, and blatantly so. People who read posts won't necessarily be in the same mindset as the poster, leaving much room for miscommunication and misunderstanding. There's also the users who aren't native english speakers, furthering possible confusion (even so, it can be hard to tell with some).
I think that a general set of rules and guidelines should be made to communicate efficiently through online forums. I'm also aware that someone has probably already thought of this and made said rules and guidelines.
All in all, I think that online communication demands slightly more imaginative empathy on the side of the readers, and more consideration on the side of the writers. With that, I'll admit that I tend to ramble about thoughts and ideas a lot - is this making sense to anyone?