There's the problem of filtering as well -- if I jump into a thread that's a few hours old on Reddit, there may already be hundreds of replies. How do I filter this? How many discussions have you been in where there were several different people all with the same response, simply because someone else had the same opinion 30 minutes earlier?

On the flip side, if you're in a small local sub, how do you get new ideas injected? It's the "joke #243" problem, where everyone's heard everything already. Until more people arrive with fresh insights and ideas, the community can become insular.