> voting on posts and karma

I agree that this is Reddit's main feature. However it isn't necessarily good. It is basically an amplification of "interest" whether that is good or bad. Assuming that votes represent general quality is a dangerous mistake.

> continually polished user interface

Ok, this must be a joke.

> Design a decentralized protocol that can handle voting/karma, while also incentivizing developers of clients.

In order to do this in a decentralized manor you need to choose who's votes you trust. This is a very interesting problem and solving it may be helpful even in the context of a centralized network.