There are already plenty of third-party Reddit apps, and this official app doesn't (yet) offer any additional functionality over those.
But there is one good reason to try out the new official app—you get three months of Reddit Gold as a "thank you" for signing into the new app during launch week.
Other than that, the official app is fine, but it's probably not worth ditching your preferred third-party app yet.
Alex Le, Reddit's VP of Consumer Product, said it best in the app announcement thread: "Our 3rd party apps are great—they've filled the void during the time when we didn't have apps. If you enjoy those other clients, please keep using them. We're going to focus on making our apps the best they can be for as many Redditors as we can. That might involve some compromises that don't work for all of our users and we're ok with that. That's why we care about staying committed to our free API."
There is a floating action button you can use to quickly make a post (when you're scrolling through posts on your front page or in a subreddit) or a comment (when you're in a thread). You can choose between "card view" and "compact view" for browsing through your front page or subreddits.
You can disable preview images, but you'll be left seeing these rather unappealing gray icons instead. You might save a little bit of data by enabling this option, but otherwise the change is purely cosmetic.
You can also enable "Night Mode" in the settings for a dark theme. While I haven't actually tried it at nighttime yet, the darker theme seems like it would benefit from more contrast between the text and the background, since even in daylight it's hard to read.
There doesn't seem be to be a way to browse /r/all at the moment—well, an intuitive way at least.
To find /r/all, head to the front page and tap where it says "reddit" in the top-left corner, next to the red Snoo logo. This brings up a list where you'll see "My Front Page," "Popular," "Following," etc. Tap on "Popular" to see a list of posts that are trending. Then you'll have to hit the three dots icon in the top-right corner to open a drop-down list and press "Sort." A "Sort by" menu will appear, and you'll want to tap the third option, "Top first."
Le says the Reddit team will continue to update the app, so hopefully they'll find an easier way to users to get to r/all.
Hopefully there will be more major improvements and features added, since right now it seems too bare-bones to pry many Redditors away from their third-party apps.