according to my opinion as user this is step back. I simply dont like ergonomy of this forum, its navigation and its various visual effect are like blending from red after opening topic and progress bars it is quite bothering. Also it wastes screen space a lot, for example suggested topics are just space eater (probably can be turned off) or text fields are too narrow.
I personally prefer PHPBB like (which SMF was). As owner of several forums I perfectly understand your reasons, but I would personally migrate to PHPBB, you can easily set moderation, you can download lots of antispam plugins, etc…and you could keep similar design.
Regarding login using social networks, this is feature is IMHO overestimated, I dont know why would I give these companies (who lives from selling personal data) information where I log in
On the other side I prefere you focus your precious time on WM, not on forum. I dont like this one, but anybody can learn to live with it. So probably not big issue.