Isn't the App Store basically just Steam but for general applications?
Application stores, which force the user to go through them (as this happens on iPhone), dramatically change the business scene for software vendors.
With "old-style" approach the user needed to go to the search engine to find an application he needs or to rely on friends' suggestions or magazine articles. This made it possible for developers to create a dozen of similar applications and compete for user's attention. They did it with SEO, SMM, and other TLAs.
If the application store becomes the only channel via which the software reaches the user, then the winner of the top position in the store's section takes most attention, leaving other developers without profit (as there's only one top spot per section usually and the developer has no way to influence the procedure of getting into the top spot).
What's even worse is that Apple's voluntary policy of accepting or denying applications for the store means that the developer doesn't know if his application is accepted until he completes it and submits it to the store.
This way, the application store turns the more or less predictable software business into a casino, where you make a high stake (needed to develop an application) and then bet that stars are positive to apple's decision to approve your application and that you win the top spot. If you lose (and chances are very high that you do), then good bye thousands of bucks and months of your life time.
Because all the application in AppStore is made for Mac. And most people prefer app store to download and install application in Mac.
I highly doubt the Mac will become a closed platform; it's a computer. Also, I read an article about this and if I remember correctly, it stated that Apple confirmed Macs will not be closed platforms. The App Store just provides simplicity to find good software for OS X quickly, I wouldn't worry about it too much. :)
The only real question is whether they can get away with doing it. The iPad and iPhone only allow distribution via the AppStore already. To me, this is a strong indication that they'd do the same with the Mac if they thought they could without losing much market share.
The only thing stopping them is the fact that only about 95% of Mac users are blindly devoted to Apple because it's "cool" -- the 5% or so who'd jump ship if they did anything too evil still represents something like $600 Million/year, which is a bit much for Apple to just throw away.
Yes and as an example, I buy ALL my games over steam. When a game is not on steam I will most likely never buy it.
The concept is the same. Easy installation and uninstallation of applications.
Steam just focus primarily on games - usually needing license checking - and the Mac Application store on applications.
So yes, they are competitors but with different customers.