Forum posts will be videoclips posted on YouTube. Of course, instead of the current low-res, blurry, jumpy webcam videos of people with too much mascara, automated software will analyze people's faces, and transfer their movements, facial expressions and gestures to their animated avatars. Sound editing software already enables filtering your voice on the fly. So you will have your opinions spoken by your avatar in the voice of your choice, and it will be as simple as talking to a webcam. tl/dr will become tl/dw (didn't watch), and that's about it.
Of course, with posts becoming video clips, spelling will become a thing of the past, and the only spelling we see will be LOLspeek on cat photos.