It was a few years ago, but I think guest accounts was a big one, as well as improved security and message search. Additional integrations/apps was probably a factor as well.

The one extra feature I wish the Standard plan had was SAML SSO - it's unfortunate that Slack is our main software that requires Google sign in rather than our SSO... but it's not worth double the price just for that feature...

Employee count and the realization that the per person cost wasn't ridiculous for the features and functionality we were to receive.

For me, free Slack accounts make the most sense for tiny startup/side projects that have a very low chat volume, where it always feels like if we hit the history limit and find our original messages needed, we'll surely also be at some level where paying for Slack would make sense.

Or, free Slack also makes sense for friend groups where you're just chit-chatting and no one really expects to need to go back and read older messages again.

But once Slack is being used for serious stuff and being able to go back and see older stuff actually saves you time and helps your team be more productive, it only makes sense to pay.