What can we do to increase security and prevent Bitcointalk suffering something similar in future?
I think Bitcointalk has a strong security system and it will be very difficult to hack the website, but regardless, let the software and plugins be always updated, theymos should also use a strong web application firewall. Lastly, fishy activities should always be set eye on, to monitor any kind of fishy activities and block it on time.
For forum users, they should avoid sharing sensitive datas or informations about themselves or their Bitcoin wallet balance.