A short list, then, of what makes Carbon a great Twitter client for people with Android phones. Not social media marketing professionals or brand managers, and not tablet users, as tablets aren't accommodated yet. But people who use Twitter to check in on the day's news and gossip, on friends using Twitter, and on whatever else is going on, this is what they might appreciate:
Holo-themed (a.k.a. modern Android), consistent-looking app
Google provides three preferred themes for Android developers. Any developer can make any Android app look pretty much any way they would like, however, and Twitter made their client look a lot like the iPhone/iPad version that came before their Android client.
Switching from Android functions and modern Holo apps into Twitter is a bit jarring. Can you deal with it? Sure you can. But I see app styling as similar to street and instructional signs in a city. Consistency lends a feeling of guidance, confidence, and confidence to guess where you're going and what comes next, while entropy and variability imply that, while the locals know what they're doing, you're own on your own.
Carbon is a fully Holo-themed Android app. Whether or not its slightly futuristic, robotic, and dark-hued looks appeal to you, you can see them as part of a whole on some half of Android phones today.
Putting the user needs ahead of branding or policy
Twitter used to show previews of Instagram photos, but no longer, following a weird "best for our users" sparring match. Carbon could care less about those fights. It shows previews of Instagram photos, YouTube links, and some web links, right in your stream. Twitter shows some of those previews on its own app, but only after you tap on a tweet.
The standard Twitter user can stand to see some rich media in their feed, because they are following only 102 people, and the majority are following about half that.
Here's a feature of Android 4.1 and later that not enough developers implement: smart, interactive notifications. That is, not just pinging a user with text that someone replied or mentioned them on Twitter, but, from that very notification, offering buttons to instantly reply or retweet or otherwise interact with someone they can even see in the image.