Being the hoster of a small LAN minecraft server, I can definitely say that I'd NEVER play minecraft (or any other similar game) without some modding ability. So, yeah, that instantly kills this FortressCraft thing for me, even though it might be promising otherwise. And well, its not.
Looking at the
page with the differences between the games I'd say that the only advantages FortressCraft might have are either shallow (Bloom, Coloured lighting, Multiple dynamic light sources, reflective water, etcetcetc... sure, graphics are nice, but just list it as "better graphics" or something), or casual-gamer-oriented (character emotes, joypad, voice chat...), or stuff that are doable in mods for minecraft (mine cart stuff, magnets, vehicles, handheld light, minions, etc.). Plus, some of the better stuff in minecraft is not going to be implemented, mainly modding support and redstone (yes, redstone is one of the better stuff, especially with mods related to it. Some crazy interactive stuff can be made, forget all this "steam power" and "better mine-carts" and "conveyor belts" the FortressCraft devs claim.)
So all in all, my current opinion is "If you want to make a game similar to Minecraft, sure, go ahead, but atleast make it better, not shallower. Also, modding."