What needs to be fixed? While you can quickly switch from feed to feed, the fact that you can't view more than one simultaneously may be a problem for some users. (You can open one or more feeds in separate windows, but this felt awkward to me.) And as of yet, while you can follow multiple Twitter accounts, there is no way to follow a Facebook account.
Finally, Mixero is currently in invitational beta mode; if you become a Mixero follower, the company will send you an invitation. The system is a bit awkward -- I had to send for a second invitation because my version didn't want to update.
Final verdict: Mixero is very much a project in development, so expect new features and changes before it stabilizes. Right now, this has the potential to be an incredibly useful and innovative Twitter client -- if the creators can add the features it needs without weighing it down.
PeopleBrowsr is a Web-based client that tries to be all things to all people; not only does it perform the usual tasks such as enabling delayed tweets and re-tweeting, but it offers a multitude of other features.
What does it do? Practically everything -- which can be a bit confusing if you don't need it to do practically everything. You can choose from three different modes. Lite is the simplest of the three but still lets you create a variety of different feeds organized into columns, which PeopleBrowsr calls PostStacks. A menu on the left side of your window (called a Quickstrip) offers the ability to click on several categories, including Followers or Searches.
Advanced mode is more feature-filled. Instead of the Quickstrip (which is still accessible if you want it), all your features are accessible by a multitude of icons that you can place at the top of your window, at the bottom or within your feeds. You can create and send to groups of users (PeopleBrowsr makes it relatively easy by offering you a checklist of all your followers); schedule your tweets and open re-tweet reports for any search. A box at the bottom of your main feed lets you sort it alphabetically or by number of followers, and more.
Business mode has a cleaner interface than Advanced mode; its standard interface shows you each tweet on a separate line, with each tweet no more than two lines. You can sort and filter by a variety of criteria -- for example, you can choose to see only tweets that include links or sort for the number of followers each user has. However, while it's easier to read, the two-line maximum means that each tweet stretches to fill your window, so if you have more than one PostStack, you have to mouse over to each, which can be inconvenient.