The idea of a Mac App Store is great... from a certain point of view. As it currently exists, the App Store has several problems and limitations which could be fairly easily removed.
1. Demos/Trial Periods/Returns: I'm not going to state unequivocally that Apple will have demos in the Mac App Store, but I will state unequivocally that Apple should have demos in the Mac App Store. As noted earlier, the Mac App Store guidelines state "Apps that are 'beta', 'demo', 'trial', or 'test' versions will be rejected." That's the same as the iOS App Store guidelines.
That should change. Demos aren't just for small developers. Apple has offered demos for their apps, including iWork and Aperture. Adobe and Microsoft have as well. They give users a chance to try out an application without having to commit to it. This concept is almost as old as computers themselves.
Apple could address this very simply, without forcing users to understand 'demo' or 'trial' versions. All they need to do is say, "You have X days to change your mind after you download an app." Maybe it's two days, maybe seven, but some clearly defined window of opportunity. Most people are going to buy and go on. Some who buy and don't like it will delete it. The App Store will know what apps you have installed. If you delete it before the return window closes, Apple simply doesn't charge your credit card. That saves the developers from losing 30% of the cost of their app on a return, and Apple already purchases some length of time before processing them.
2. Upgrade pricing: Right now, developers have no way of giving free or discounted upgrades to existing customers. This has been a problem that Apple was going to need to address even without a Mac App Store. Sure you may be willing to spend $40 on OmniFocus for iPad, but are you going to be equally willing to spend $40 for version 2? The folks at Iconfactory were skewered over a $5 upgrade for Twitterrific, can you imagine what OmniGroup is going to face when their apps come up for upgrade?
Again, this is a simple fix: Developers should be able to set two prices: one for those who have bought a previous version, and another for those who haven't. Apple knows your purchase history, and should simply display the appropriate price in the App Store. It's not rocket science.
Apple doesn't do upgrade pricing for its consumer apps. If you go to buy iWork or iLife, you're paying the same price regardless of whether you've been an iLife user since the beginning or whether you're just picking it up. Logic Express, Aperture, Final Cut Express, and Final Cut Studio all have upgrade pricing.
These are both problems which existed yesterday before there was a Mac App Store, but the existence of a Mac App Store just makes these two problems all the more obvious. Both could be easily fixed, but it's entirely up to Apple to fix them. Doing so will benefit not just Mac users, but iOS users too.