Basically everything. DeFi wasn't even really a thing back then - Ethereum had zero TVL when the rollback happened. Smart contracts were a brand new concept. Competing SC chains like Solana weren't even in the idea stage yet. It also had only one client. Now there is $50B+ in TVL and nine clients and a rollback is just not going to happen.
It's ancient history. It's like still criticizing Apple for how they handled Copland OS.