It is rare for business software to arouse emotion besides annoyance. But some positively gush about how Slack has simplified office communication. Instead of individual e-mails arriving in a central inbox and requiring attention, Slack structures textual conversations within threads (called “channels”) where groups within firms can update each other in real time. It is casual and reflects how people actually communicate, eschewing e-mail’s outdated formalities, says Chris Becherer of Pandora, an online-music firm that uses Slack.