You can choose to install apps from anywhere, just as youdo today, or for maximum security you can set Gatekeeper to allow only apps from the Mac App Store.
If you were already enjoying the fruits of the early builds of Raspbmc, you'll need to do a complete re-install but that shouldn't be too much trouble since you probably store your content on a network drive or external USB.
In other words, while you can install an app like Photoshop on a full Windows 8 machine, you can't do it here, nor any other Windows application written since the dawn of the OS. Additionally, we have our doubts about whether anyone going forward will bother to write desktop apps versus those more optimized for running in the de facto Live Tile interface.