That is an important characteristic of online communities: someone with a bad reputation can usually (depending on the application) throw away their old account

That is an important characteristic of online communities: someone with a bad reputation can usually (depending on the application) throw away their old account and start over with a new identity; therefore, having no reputation is more-or-less indistinguishable from having a negative reputation. That's not really a problem per se, it's just a characteristic of the problem domain to consider when implementing a reputation systems (and in particular, in deciding how much leeway to impact the system to give to new users).