Vitalik Buterin stated that basically, there's two main classes of regulatory policy goals: consumer protection and making it harder for baddies to move large amounts of money around. The issues around the second question are concentrated not in defi, but in large-scale crypto payments in general.
Regs on defi frontends that could be more helpful may include: (i) limits on leverage. (ii) requiring transparency about what audits, FV or other security checks were done on contract code. (iii) usage gated by knowledge-based tests instead of plutocratic net-worth minimum rules.