Because I love demons and alternative casting costs. :P

Um, dropping one of these on turn 1 seems a bit powerful. And you bet I'd mulligan five times to do so.

An effect worthy of mythic, right?

No. An effect that shouldn't be printed. The alternate casting costs on demons are always something you can't do on turn 1 without a lot of effort. Compare Demon of Death's Gate: you need three Ornithopter/Memnite to turn 1 him. Which is a fun possibility. This on the other hand would be a dreaded inevitability.

You need to add/replace something such that the ACC needs more work. Exiling half your library is utterly, utterly irrelevant.

Now if it was exile all but the bottom (say) 4 cards of your library, that might be more interesting...

New and improved.

Also, you couldn't Ornithopter/Memnite out Demon of Death's Gate. It has to be black creatures. :P

Technically, you could with Darkest Hour, but that's a bit trickier to pull off.

Yee-hee-hee-owsers. That's... really cool. I don't know whether it's printable now, but it doesn't strike me as "alarm bells, never do this" any more.

This is actually probably most useful as a last-ditch desperation blocker. If you've held back a couple of land in hand, you might live long enough to get to attack with him.

Oooh. Still kinda too evil to be allowed in a casual environment - but for competitive would you DARE drop him turn 1 now, knowing you'd only get another few turns? And knowing whether or not you've got a counterspell for the opponents removal in those turns, and is it going to be in your hand in time...

I kinda like. Very much a "If this is in environment, you'd better know it exists", which I have a whole different set of problems with, but... yeah. At the level where I'm now going "Well, it's only 6/6 and not even completely unblockable..."