Architectures for Homebrew OSes?

WalterGR asks: "The recent article about Athena and discussion of other home-brew OSs has really gotten me interested in the subject. I'd like to try my hand at it, but I don't want to make just-another-[insert your favorite platform] clone. I'm not striving for compatibility, as I don't want to be hampered by legacy systems. Because compatibility won't be an issue, I'm at liberty to choose any architecture. Which one would be best for a brand new OS? The PC architecture seems to be a mess of backwards-compatibility baggage and afterthought. What is the best alternative, considering speed, availability of hardware, cost, availability of documentation, design, and so forth?"