[JoshMandel]

Obviously subjective but since comments mention UI: I love the web/desktop UI. It's got great power user features including well designed keyboard shortcuts. https://zulip.com/help/keyboard-shortcuts for details (my favorite is "n" for "next topic", which I use every morning to catch up on conversations).

[sundarurfriend]

n for next topic, j and k to move up and down within a topic, + to add a thumbs up to the current post, : to add any other emoji, and t to get a list of Recent topics so you can decide which ones to catch up on first. All these make Zulip so pleasant and breezy to use.

[stavros]

Plus it's fast, Slack is a vat of molasses and I hate my life every time I have to use it.