I currently run SMF but have used phpBB, punBB and FluxBB. I really loved the speed and simplicity of FluxBB.
Anyway, for SMF :
- Pros : fast and light on the server, has all the functions you need built in. No need for plugins that will sooner or later become unmaintained. It's stable and reliable.
- Cons : code base is so old... Developpers are gone, themes and plugins are a nightmare. Admin interface is very complicated. Not mobile friendly.
I vastly prefer having recent topics on the front page and categories / tags in the sidebar like Flarum. I just hope there will be a way to stop topics from a particular category / tag to show up on the front page.
I also hope that important plugins (like attachments, merge / split topics, importers) will be maintained by the core team.
I think private messages are not that important, as long as you can send emails from the Flarum interface (like in FluxBB). It's simple and works really well. I hope it will be in core.
I also hope that auto embed of images URLs (and why not YouTube vids but that could be a third party plugin) will be there. And that the attachement system will be just a simple upload form that gives you the url of the image when it's uploaded (like an image hoster). You would then copy / paste the URL where you want in the post. The actual displaying of the image would then be taken care of by the auto embed functionnality. Simple, effective, can survive a migration to another forum script etc. That is my number one dream for Flarum