The Mac App Store was a minor revolution when it first started doing business back in 2010. Like the iOS App Store, it lets you easily buy, download, and update apps that you can use on multiple Macs. It also serves as a conduit for updates to Apple software: both the operating system and Apple's apps, such as iTunes, Pages, Xcode, and others.
For many users, it's a great way to buy apps. It offers a single location to get software, and you pay Apple using your on-file credit card or gift card balance. You don't need to worry about giving your credentials to an unfamiliar website, you don't need to store serial numbers, and all your updates come through a single conduit.
But there are plenty of reasons to not buy from Apple; a number of key developers have pulled their software from the Mac App Store, because of the impossibility of having demo versions and upgrade pricing. (This article gives a good list of pros and cons of buying from the Mac App Store.)
The Mac App Store needs some work on user-friendliness. It's easy if you want to buy an app, but when you want to find something you've already purchased—say when you get a new Mac—it's not always simple. The Mac App Store app has a Purchased tab, and it shows your purchases only in reverse-chronological order. If you remember all or part of the name of an app, you cannot search this list, as you can in the iTunes App Store. If you enter something in the search field at the top of the window, it searches all the apps in the Mac App Store.
One of the useful features of the Mac App Store's Purchased list is the ability to hide items you no longer want to see. You may want to do this for a free app that you tried but didn't like, or even a paid app that is no longer updated, or doesn't work as well as you hoped.
Normally, when you view an app in the Mac App Store that you've already purchased but that's not on your Mac, you don't see a price below its name; you see the word Install. But if you ever want to re-download a hidden app app, and search for it in the Mac App Store, you see its price, and may not recall that you actually paid for it.
(If you ever want to un-hide an app, choose Store > View My Account, then check the Hidden Items section. You can "Unhide any hidden, redownloadable purchases.")
Another quirk is that Apple, or a developer, can pull an app from the Mac App Store at any time. If this happens, you won't be able to re-download it, even though you may have paid for it. And there's no indication when this happens. Curiously, you can download some apps that are no longer for sale. If you've been using the Mac App Store for a long time, go to the bottom of your purchased list. Hover your cursor over some of the icons. If the cursor doesn't change to a hand, then there's no longer a listing for that app in the store. But the Install button still lets you download it.
Admittedly, these are minor niggles, but as the Mac App Store is now more than six years old, lots of Mac users have long Purchased lists, and may confront some of these issues. It wouldn't take much to fix these; just treat the Purchased list the way the iTunes App Store does. Some small changes could make the Mac App Store a bit more user friendly.