The format and discoverability are the biggest problem. Any conversation is hidden away in accounts tweets. If you don't know the specific account then you won't know what's being talked about. There's also no community unless your already famous. It's essentially anonymous.
Reddit I think has the potential for the best discussion platform. The people are the biggest problem, and upvotes are a good idea, but doesn't seem to work in practice
IMO discoverability is an anti feature. Having random people drop in constantly means you can never have a real community or users that are even familiar with the rules.
I love things like IM groups/discord servers because they are somewhat hidden and hard to find. You get invited by a friend or see a link somewhere and join and when you do join you tend to stick around for a while.
Reddit I think has the potential for the best discussion platform. The people are the biggest problem, and upvotes are a good idea, but doesn't seem to work in practice