I've only been running Twitter's new Lists feature for a few days, but it's already changed my behavior in one important way: I'm unfollowing a lot of accounts. Of course, my use may not be typical, but I think it's going to cause a LOT of people to unfollow a LOT of accounts.
Don't worry, [this is good].
When you create a new list, you can mark it as either public or private. If a list is public, other people can follow it. Twitter set this up so that when you follow a new list, the people on it do not show up in your main timeline. In this respect it's different than the way grouping works in most Twitter apps. (Groups typically let you filter people out of your main timeline, so that you can follow a subset of users more closely.) Twitter lists are kept completely separate from your timeline, so when you follow a list that has, say, 40 users on it that you aren't already following, their updates only appear in the list itself, and not in your main feed. Likewise, when you create a new list, you can add people to it who you don't actually follow, and then follow them only via the list.
This solved a problem for me.
I follow somewhere in the neighborhood of 700 accounts. While my Twitter muscles get bigger all the time (for a long time I found anything over 100 disorienting) that's just too many accounts to reasonably keep up with, and it means I miss a lot of stuff.
Those 700 tend to fall into two main categories: accounts where I want to see every single update in real time, and accounts whose updates I want, but don't neccessarily have to read the tweets as they are posted. I still want to follow the latter, so I can't unsubscribe, but the updates tend to clutter my feed. This is especially true for automated or formulaic Twitter accounts, like news accounts that use Twitter like a headline feed, restaurants that publish daily or weekly specials via Twitter, or even bot accounts like the low-flying rocks feed that are interesting to me but not neccessarily timely. I can now split these off into separate lists ("News" "Food and Drink" "Robots") and peel them off of my main timeline.I've already begun doing this.
I've also found myself unfollowing people, and adding them to lists. Perhaps they tend to point to trends, interesting links, or weigh in only (or even primarily) on specific subjects. In my case, many of the people I follow tend to fall into what I think of as a required reading category. I feel like I need to keep up with them for work purposes, but I don't want to see their updates as they happen. Now I can assign these people to their own list, check it once or twice a day, and again free up space in my main timeline.
Eventually, what Lists will accomplish for me is to allow me to filter Twitter into two large groups. The first group will be composed of people whose every update I want to read immediately. The second group will be subject specific, with many subgroups. And by peeling those subgroups off, it's going to make it much easier to not miss tweets from the people who I most care about. Furthermore, since I now have to make an effort to read the subject-specific Lists, I'm going to look at those tweets more closely.
There are going to be a few net effects of all this. First, the most immediate short term effect is that I'm going to unfollow a lot of people. Sorry if this is you. But the more important long term trends are that I'm going to actually receive updates from even more people and businesses and robots than I was before, and I'm going to be able to pay better attention to everyone, regardless of whether I keep the account in my timeline, or on a list.