But it's also the fact that while it's easy to get started, the value and dependence on Slack increases rapidly by your team's investment in using it.
Slack takes no time at all to do an initial team setup so you can just get started and as mentioned nothing needs to be learned to dive right in. Users are rewarded for discovering new things and further rewarded by cool points for sharing the tricks and secrets they uncover with others.
The ability to search and find a file or a key part of a conversation that happened a year ago faster than you could an email. This feature only gets better the more your conversations happen in Slack — you'll never lose anything again.
In terms of features, many of the best customizations involve apps working with Slackbot and Zapier. Setting up these custom features takes time and tweaking to get them exactly perfect — switching costs you won't want to deal with a year from now once they've become part of your routine.