There is no mechanism to ignore a user, mute a user, or temporarily suspend a user if they need a timeout. The first two are completely impossible, while the latter today is manual / mostly possible using undocumented APIs and mechanizing of HTTP requests. Users reporting spam is done by them sending a message to an admin, or in the admin channel, and requires someone to manually intervene and moderate the message. So we effectively need to aim for follow-the-sun moderator coverage, which is a bit painful.
In terms of usability and accessibility, some of the things Slack does (e.g., threads) are not super-inclusive or accessible to users who use screen readers. Today, based on feedback I've gotten, IRC is still superior for users who rely on screen readers due to some form of visual impairment.
Whichever product they go with, it'd be nice if they provided an authenticated IRC gateway which would allow this class of user to have a better experience using the platform as they could use one of the text-based IRC clients that has served them well.
For everyone who is saying they hope Slack isn't the choice, I'm fairly confident with these issues today they won't pursue Slack. If anyone from Mozilla, or any other org, would like to discuss some of the challenges we've had using Slack as an open community I'm more than happy to schedule time. I'd like to help you provide the best experience and community for your users.