I don't think these sorts of corner cases can be avoided. When they come up, my preferred approach to resolution is group consensus led by (but not necessarily determined by) the GM. In the case of the prone ooze, I'd probably disregard the penalty to ranged attacks. In the case of the steam, I would want to think more about the nature and source of any fire resistance before letting it apply. In the case of the halfling using Come and Get It against the gnolls, I'd ad lib in some explanation for why all the hunters choose to close even though they have an obvious and overwhelming advantage against the halfling from their current position.
Here would be my "rulings" on such cases.
1.) The ooze isn't knocked prone, the penalty is negated. Sorry, hope that wasn't an encounter power; next time don't trip to knock an amorphous blob prone.
2.) Steam-heat is heat. I'd rule Fire resistance works.
3.) I'd say that CaGI fails. The gnolls (assuming they're not stupid, I don't know the average gnoll Int off the cuff) have a superior tactical position and the halfling is at horrendous disadvantage. The power isn't wasted, but the halfling hurls threats and insults, the gnolls respond with some arrows that miss, and the halfling reconsiders his tactics.
"But Remy!" I hear the crowd shout out, "That's not fair to arbitrarily decide a power doesn't work! You're robbing the player of his ability to affect the fiction, making his choice of power, sub-optimal, yadda yadda"
Here's my RBDM response: So what?
Fireball's don't work underwater. Neither do slings. You can't burn a fire-elemental. You can't convince the paladin-lord you're his bastard love-child. And I don't care if you have skill focus in Athletics, you're not convincing the king to give you a boat during the skill challenge by doing an absurd number of push-ups!
At a certain point, the fiction doesn't bend enough to allow it. I can craft some pretty convincing rationales and examples, but I seriously doubt any of you would actually believe I'm actually 22 foot tall space alien from Zogtor.
At a certain point, the DM has to say "No." This flies contrary to 4e's "Say yes and explain it later" mode of operation, and it jars me to no end. The thing that killed 4e for me, more than ADEU, more than "the math", more than any single mechanical or fictional element 4e introduced, was the off-hand way the game took the ability for the DM to control his world away. An Ability like CaGI takes the DM's right to play monster's intelligently away from him and reduces them from rationale beings in the world to pawn on the chessboard. The game has to have limits, and I actually hope Next puts more limits back in, so I that never have to have the "the rules say the ooze is prone" argument again.
There are corner case, and then there is rationalizing an onrushing mage charging, dagger first, at a full-plated fighter with a greataxe...