A beholder would really fit the bill. It's a complex monster, from a high-ish level, gives us a real sense of how simple they're making the game.
what is this thing of Mike's rust monster? did they nerf or change it so that it has to be house ruled by us to make it make sense?
A bit. The argument was that loosing your equipment is an IMMENSE killjoy, and that the rust monster isn't a challenge except for the fact that it can destroy your equipment. So the Mearls redesign beefed it up physically, while weakening it's rust attack. So it became a bit more dangerous in combat, and the rust became kind of a general weakening effect (I just think a -2 penalty or something) that went away after the encounter.
My main issue was that the rust magically vanished for no reason. Others took issue with the fact that the rust attack was weakened. Others had problems with the initial argument, seeing the focus on items as an excuse to coddle crybabies.
My guess is that, since 4e has less reliance on equipment, the rust monster might not loose much potency. It might not be as able to affect magical equipment -- your ancestral magic sword won't just poof away -- but it probably will be able to soften, weaken, and eventually destroy other stuff. I don't think the attack will be one-hit-poofing, because that's basically Save-Or-Die, and I don't think that the effect will magically go away (a Craft check might be enough to fix it, though).