An Amiga must have Paula, Denise and Agnus (or Alice/Lisa in case it's AGA). An Amiga is an Amiga even if you run Linux, Amix or NetBSD on it.
An AmigaOS-compatible computer is something that runs some incarnation of AmigaOS but it's not an Amiga (doesn't have Amiga custom chips): it cannot run natively Amiga software that bypasses AmigaOS.
FPGA machines are Amiga-compatible reimplementations but are not Amiga machines, although in any case minimig is more "Amiga" than any PPC board with a sticker.
Example: A DRACO is an AmigaOS-compatible computer that runs Amiga software but it's not a real Amiga. A PPC board with an Amiga sticker won't turn into an Amiga even if some people insist on calling it "Amiga" just like a parrot won't be a dog even if it barks like a dog and hates cats and you call it "dog" and put him a small dog collar.
PS: An AmigaOS-compatible machine can be a wonderful machine, you could even run Amiga software on an emulator, you could run recompiled AmigaOS software but it won't be an Amiga, and sticking calling a standard ppc board "Amiga" is as silly as pretending&claiming that a "cat" is a "dog", both can bring you good company as pets but aren't the same.
