So what are the new limits if links, usernames etc. do not count against the 140 character limit? It will hopefully not be possible to create "monster" tweets with thousands of @ notifications and/or hundreds of images...
As for images, here's a limit of four per post, and Twittwr groups multi into a display with one URL. Videos, Vines and polls are capped at one.
It's possible to have many, many usernames in a reply, but I don't know if there is an upper bounds.