The most immediate way Slack is more "flashy AIM" than "flashy forum" is that threads don't bump back to the top when they get recent activity.
If someone replies to someone else's message in a thread and it scrolls off my view, they could have a 50+ message conversation in there that I would never see.
In a "traditional" forum, if a topic is that hot, it'll stay on top of the topic list. Even in a more threaded forum like HN, where ordering is done separately, when I refresh, I'd see there were more replies on a hot sub-thread, in a way that can disappear entirely in Slack.
If someone replies to someone else's message in a thread and it scrolls off my view, they could have a 50+ message conversation in there that I would never see.
In a "traditional" forum, if a topic is that hot, it'll stay on top of the topic list. Even in a more threaded forum like HN, where ordering is done separately, when I refresh, I'd see there were more replies on a hot sub-thread, in a way that can disappear entirely in Slack.