Do you think when people come they should arrive on Latest like currently, or rather land on Categories?
Not very easy to answer.
In Discourse I think may be better Latest.
I think latest. That way new users can easily jump in to relevant discussions (for ML type threads) or can easily help new users asking for help with OMA (for forum type threads), rather than having to start digging to get to this content.
Registered users can change this (i think) if they’d rather just see the overview that the categories view provides.