Nork said:
Having a monster that follows most 90% of the rules of the game, and then has an ability that is wholly outside of any mechanics or rules other than 'he can just do it because it feels appropriate for him to do so, there is no greater rules architecture that justifies it', provides a good baseline of consistency (he follows 90% of the rules), and a 'cool' factor where he breaks 10% of the rules. The DM serves to keep the 10% at an appropriate level.
But what about when it's just as cool to have the PCs do it?
Here's an example of that kind of system done poorly, and I am part of the team that did it. I worked on the S&S Gamma World. I was assigned the monsters, mostly converting a lare subset of the original critters and adding a few originals. Since the PC mutation rules hadn't been written yet, I hand-converted all the old Gamma World mutations I needed and gave them to the appropriate monsters, in some cases trimming out a lot of hodge-podge powers to help give each creature more of a coherent focus (uhm, sounds familiar all of a sudden...)
The problem was, the PC mutation rules were very different -- and much, much, sparser -- than the original GW rules, due to those designers having a different 'vision' of GW than I had (I was going for as much of the original gonzo spirit as possible, tied to a richer backstory and more of a reason for most of the creatures which existed; they wanted to get much further away from classic GW, but that's another issue). The result was that the monsters had cool mutations which the PCs
should have been able to have, but
couldn't. I eventually got to extracting them all from the monster lists and writing them up for PC use, but the frustration at the design disconnects in the original PHB tainted the overall reception of the game, and I still get flamed on GW lists over it. So it goes.
Fact is, when a monster does something Really C^h Nifty, and there's no obvious reason why a PC can't do it, too, a player will ask, "How can I learn to do that?" "Wait for the appropriate supplement" isn't always the best answer, though, given how much has to fit into the 4e PHB, it might be the only viable one. Probably won't be too hard, once we see the way powers are designed and scaled, to figure out what level of feat/talent/power this trick is and offer PCs a chance to buy it. ("Well, first, you need to find a friendly bugbear to teach it to you...good luck with that.")