So. Fog (vapors), heavy fog and darkness are obscuring terrain that block line of sight. Fog (light obscured), heavy fog (heavy obscured), darkness (totaly obscured). True?Obscured terrain provides concealment and blocks line of sight if a target is far enough away from you. However, it has no effect on movement.
Lightly Obscured: Squares of dim light, fog, smoke, heavy snow, or rain are lightly obscured. Taregt have concealment.
Heavily Obscured: Squares of heavy fog or heavy smoke are heavily obscured. Adjacent creatures have concealment. Far away are not seen.
Don't forget that the Cloudkill on 19 Daily Power don't says about total concealment just like Stinking Cloud dosen't. It just block line of sight.The cloud blocks line of sight, providing total concealment to creatures inside it. Any creature that enters the cloud or starts its turn there takes 10 acid damage. The cloud lasts until the end of your next turn, or you can dismiss it as a minor action.
It's funny that you'd bring that one up because to me it seems more like a power that is exactly like stinking cloud, except they bothered to clarify it.It's 23 Encouter Spell "Acid Storm":
Wall of Fog:Obscured terrain provides concealment and blocks line of sight if a target is far enough away from you.
And when you add this 23 Encounter well. You right. That will be the best solution for Stinking Cloud --> heavy obscured (total concealment).The fog grants concealment to creatures in its space and blocks line of sight.
In general, it's a hard spell to counter. I've been trying to think of how I would do it myself, short of "always including a dispelling mage" or "always using enemies immune to poison (and/or necrotic, since the gloves work on it too)" - because those are so specific that it'd be pretty cheap for a DM to constantly throw guys like that at a party just to kill one power. (And un-fun for the player.) The only thing I can really come up with is to exploit its two weakest points: the damage it deals per round (low) and the concealment it grants.
@Kelanen well thats changes everything but what if power alows you slide and you just put the target into zone out - in - out - in? So rule. Just one dmg per slide or allow it?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.