The only big thing that Slack has over IRC is being able to catch up on what was said earlier.
However, to be honest, for open source projects, 'chat' of any kind should be for quick and unimportant discussions, or help. Important decisions should be hashed out on mailing lists so that people in other time zones can participate, so they get archived, and so people can take their time to write something that is well-reasoned.
However, to be honest, for open source projects, 'chat' of any kind should be for quick and unimportant discussions, or help. Important decisions should be hashed out on mailing lists so that people in other time zones can participate, so they get archived, and so people can take their time to write something that is well-reasoned.