What's interesting is that, even though they're not well-defined, there certainly are specific communities on Twitter. There's tech, and sports, and science, psychology, politics, music, etc, and each of those can be further divided. I'm sure that if Twitter put in the work, they could suss out all these communities based on who is following who and what they tweet about. (In fact, I'd be surprised if they aren't already doing that.) They could then make these communities accessible to the public.

Twitter currently has lists, but the problem is that these lists aren't crowdsourced (to my knowledge). They're maintained by individuals, which is unsustainable, because one person can only do so much work keeping a list accurate and up-to-date.