Sure, the Mac App Store is a good idea in theory. Just like the Linux repositories that came before it, it provides a one-stop shop for all your software needs. There's just one big problem: Apple made it.
You'll Have to Re-Purchase Many of Your Apps
One of the biggest questions on everyone's mind is "can I take advantage of the App Store if I've already purchased an app?" The answer is no. Developers have confirmed that there isn't currently a way to migrate your purchases to the App Store for free, so even if an app shows up as "installed" under the Mac App Store, you still won't be able to get updates for it. You'll have to re-purchase an app to get full App Store support. Sure, some developers are trying to work around this, but most are stuck with no easy solution. If you've spent years using Macs and purchasing good commercial software, it's unlikely that you'll be able to take advantage of the App Store anytime soon without shelling out more money — basically, those that use free software are fine, but those that pay to support good developers are doing so with no thanks from Apple.
You Won't Be Able to Test Trial, Demo or Beta Versions of Software
One of the best parts about commercial software is that you are often allowed to try a piece of software for 30 days before buying. That way, you know whether the software is worth your money or not. Furthermore, those of us that like to live on the edge can test out beta versions of software to help the developers find and fix bugs, and in return we get a sneak peek of upcoming features. If you use the Mac App Store to download software, however, you will not benefit from either of these.
One of the restrictions on the Mac App Store is that no pieces of software will allowed to be labelled as "trial", "demo" or "beta" versions. This rule is annoying enough in the iOS store, but it's worse on the Mac. Most mobile apps only run for a few bucks a pop, so if I hear that an app is really good or I think it sounds like something useful, I can usually risk spending $1.19 to try it out. Desktop software tends to be more expensive, though. I don't want to spend $40 just to see if an app is the right one for me — I'd like to be able to try out all its features (meaning no "lite" versions) before buying.
It Will Be Harder to Get Support
Forget that Apple's taking 30 per cent of each developer's pay cheque, but they're also making it harder for developers to give us good support. Before, you could just download an app from a developer, try it out and get support directly from them. Now, though, with Apple as a middleman, the developer doesn't always have as much power to fix problems that arise. They can't release quick updates, since all updates have to be approved by Apple. They can't fix any problems you have with downloading or purchasing an app (that's all on Apple's head). Having Apple in the middle of the developer/customer relationship is just going to muddle things up and make it more difficult for everyone involved.
Apps Will Still Be Heavily Restricted
The other restrictions on apps are just as ridiculous as on iOS, although once again, they have much more weight on the Mac. There's a pretty hefty list of them, that are all going to basically require developers write toned-down versions of their apps, most notably:
No paid upgrades: One of the great ways developers reward long-time users is by offering updates to them at a discounted price. This won't be allowed in the App Store — if a developer wants to have a paid upgrade, they'll have to submit it as a completely separate app, and everyone will have to buy it again at the same price.
No background processes: Apps aren't allowed to keep any code running in the background after they've been quit. So, for example, Apple's own FaceTime has the convenience of staying out of your way until you get a call, but you're going to have to keep any other video chat program fully open and minimised at all times. That doesn't seem fair, does it?
No imitating the UI of other applications-: Apps aren't allowed to imitate other pre-bundled Apple programs. Not only is this something most users want (how many times have you heard someone say they don't like an app because it doesn't "fit in" with other Mac programs?), but it's really vague. Does this mean no Adium, since it often imitates iChat? Where's the line? I can see this being an annoyance (or at least confusing) for both developers and users.
Many Apps Just Plain Won't Make it To the Store
The above are just a few examples of restrictions that will be placed on software inside the App Store. There are, of course, much heavier restrictions that basically eliminate any possibility of some apps getting accepted, like:
No root permissions: No apps are allowed to request root permissions (even with the user's consent), which means no backup software or anything else that needs access to system files.
No programs that download other programs: This is also pretty vague, but does this mean no other browsers? Does this mean no FTP clients, or anything else you could use to share files (like Dropbox)? Or are we just talking about downloading and executing code?
Will we see free and open source software?: This one's just speculation right now, but we saw what almost happened with VLC in the iOS App Store: VideoLan decided Apple's closed-off model might violate the GPL. It doesn't seem there's been a consensus on this yet (and VLC is still in the iOS app store), but developers that really care about software being free (as in speech, not as in beer) may just stay away entirely.
Of course, you aren't required to use the Mac App Store. And you should take advantage of that fact. From the looks of it now, the headache that this is going to cause far outweighs the minor benefits. Sure, it's a one-stop shop for all your software — but honestly, I'll stick to scouring Google if it means I can bypass Apple's walled garden. If half the apps I use won't even end up in the App Store to begin with, then what benefits am I reaping by using it?
What I really fear, though, that the store will have repercussions on us that don't even use it—how many developers do you think are going to code two versions of their apps, just to keep us old-fashioned users reaping the benefits of an free market? I wouldn't wager that many would.
Of course, you all probably have your own opinions, whether you agree with us or not. So share your thoughts with us in the comments!