Apple and other do this by simply numbering the names of apps. They don't allow you to specify special "upgrade" pricing, but the effect of this was that developers no longer really have full retail pricing and everything is just set to the upgrade price.
Logic Pro 8 for instance used to retail at $499. The upgrade price was $199. Now Logic Pro X on the Mac App Store is just $199 regardless of whether you are first time user or someone who had the previous version.
- The store interface and navigation are also much slower than the iOS counterpart.
I haven't really found that the Mac Store is any slower. I've found that they are both slow.
- Not 100% sure on this one: You can't download older app versions if your OS is no longer supported.
I don't believe it will even show you newer versions of the apps as long as the developer properly specifies the minimum OS version.