The plus side of sandboxing is that it means, in theory, that your apps will become safer and more trustworthy: Your Mac prevents them from accessing files they shouldn’t access. But that security comes with a price, at least in some cases. Some developers say that sandboxing will force them to remove features from their apps—or, in some cases, to pull them from the Mac App Store entirely.
Good article that covers sandboxing in a bit more detail. Some apps (Alfred, Moom) have already told their users that the Mac Store versions of their apps are basically dead.