The killer feature of slack is that alerts are highly configurable and work properly across devices - it lets you eliminate unnecessary and duplicated alerts
Sure, but there are still two major problems IMHO. Firstly, there's no practical and intentional workflow to postpone things (an equivalent of "mark as unread"). Secondly, because a majority of people responds to notifications immediately, it becomes socially unacceptable to not do it in a timely manner.
If you've read a message, why aren't you responding? It's likely not a problem with Slack that you cannot tell one of your coworkers that you will look at something or do something later.