does "Shifty" allow them to skulk in thedarkness or set traps and ambushes?
They can split their move on either side of a standard action. This can sometimes help.
Does "Shifty" allow them to swarm over weaker opponents and hide from stronger ones?
Yes, because it significantly facilitates both flanking and retreating.
Does it help them to sneak up and attack foes?
To an extent, because of the movement flexibility it grants. But not as much as the above.
Does it allow them to attack in a sudden rush?
Yes, because it allows withdraw + charge.
What exactly in that description of their tactics does "Shifty" even represent?
Well, I've given a couple of examples there.
My question is - has anyone running kobolds ever had trouble interpreting or using the Shifty power? Has anyone running hobgoblins ever had trouble interpreting or applying the Phalanx Soldier trait? If they have, I'm baffled, but obviously 4e is
not the game for them.
If you want to run this argument, though, that the words used to describe NPC powers aren't adequate to the task of running them and conceiving of them in the fiction, I think you'd be on stronger ground if you looked at something like the pact hag. Understanding what is going on with some of those powers is, in my view, a bit more intricate - although when I used a pact hag it didn't take a lot of thought to decide how I would run them - for example, after a failed check in a negotiation skill challenge I told the player in question that the hag suggested to his PC that he should move from where he was standing to place XYZ, and that he did so. (Not long after, the trap door was opened!)
The problem here, as I see it, is that 4e is chock full of keywords that don't, apparently, mean what they mean in the dictionary. I can be "bloodied" by words, influence geniuses and programmed things to Come and Get It, push dragons or giants around, have square fireballs that avoid friendly figures, knock a snake prone, etc., etc.
Every time how a PC power is questioned as it related to common sense, the chorus is that the words don't necessarily mean what they mean in common usage.
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But, please don't tell me that the description is both important, and has no effect on when the power applies.
Haven't you answered your own question? Come and Get It is a PC power. The burden of description falls primarily on the player. The PHB (somewhere around p 60) states that the player may characterise the power as s/he sees fit, provided that it coheres with what is going on in the fiction.
Keywords are a different matter again. "Fire" as a keyword means fire. Heat. Burning. This is what tells us that fire spells can set things on fire whereas weapon attacks cannot. There is a discussion to this effect somewhere in the DMG, I think around p 40 (where it talks about attacking objects).
Shifty is an NPC (kobold) power. Why would a GM who is wondering "What is a kobold, and what is going on when it shifts as a minor action?" ignore the word used, and the implied description? Of course it's the GM's prerogative to reskin, but why would a GM who is wondering what the original skin is ignore what is stated?
:If those terms have meaning within a general context, fine. If not, fine. If you expect to shift from one to another, as needs suit, that's shifty.
I'm confused as to what you think the debate is about. Why would a GM wanting to know what a kobold is like ignore the fact that one of its powers characterises it as shifty? And that the mechanical consequences of that power flesh out what this shiftiness consists in, namely, that it can swarm foes, withdraw from melee easily, etc.
This has nothing to do with whether or not play should grind to a halt when the player of a fighter uses Come and Get It, or knocks a snake prone. Whatever puzzles those situations evoke (which, in my view, are grossly exaggerated - it might be undesirable for some to run a metagame heavy RPG, but the idea of such a thing is hardly puzzling), they don't resemlbe the puzzle of "What is the monster like, and what does it do?"