There are tons of things that makes using Slack "an experience" - tiny bits of things that are not really "necessary" but just add that little bit of polish. This is gamification at it's best - when it stops being a excuse to add some points and silly badges and developers use real, practical things they generally use to build good games and apply it to non-game products.
The end result is an experience, as opposed to just "features". Hipchat doesn't get it. Neither does Hall or any other startup in this space.
Slack's only real threat, IMHO, is Whatsapp. Whatsapp has a brilliant mobile experience which works offline too - Slack is weak there and I don't know why they can't have a mobile app that works properly offline. With Whatsapp coming to desktop (although via a mobile go-through), whatsapp is suddenly a bit more usable for a team.
Now if they introduce a whatsapp API which works without having a phone number, it can rapidly make it an amazing alternative to Slack. Don't know if they will be going behind the B2B market though.