Perhaps with the odd build and the odd NPC. But not in general.
Yes, in general... and no it's not an odd build or odd NPC... it's a normal ranger and a monster of equal level I am comparing it to.
A first level fighter in Moldvay Basic, for example, cannot kill an 18 hp ogre in a single blow of his/her two-handed sword (max damage 13 hp). A first level fighter in 4e cannot kill a 30 hp humanoid in a single charge (max damage 12 for the crit +5 for 20STR +1 for expertise +2 for whatever miscellany I'm not taking account of gets you to about 20 or so). The cleverness of the charge, or the player's narration, doesn't change this. The GM, rather, is expected to narrate what occurs around the mechanics, with an assumed "I try" out the front of the player's description of the foe's decapitation.
A first level fighter in Moldvay can kill most anything with equal HD in one hit... And a 4e Fighter is now more based around defending than attacking... but the Rogue and Ranger would disagree with your assertion that a PC cannot kill a normal monster with one attack. This assertion is just flat out wrong... If the PC rolls well enough to hit and enough damage... the decapitation is not a "try". So with a good enough roll the PC can kill a monster in one round... what he can't ever do is complete his objetive in a skill challenge with less than the pre-determined successes. As another example of how one can end a combat in one round, in the right situation the choice to narrate pushing an enemy off a cliff and succeeding at it mechanically allows one to circumvent the normal attack and hp depletion routine of combat.
Similarly in a skill challenge. When the player explains what his/her PC is doing, and how it will brilliantly accomplish all that needs to be accomplished to resolve the situation, the GM's job is to narrate the consequences and ensuing complications in a fashion that (i) reflects the result of the skill check as framed, and (ii) reflects the location of the success/failure within the overall challenge, and hence either keeps the scene alive, or closes it, as required.
So again, the actual plan, feasibility, effectiveness, situation and preparation, ultimately have no bearing on success or failure... only whether one has rolled a pre-determined number of successes or failures actually matters.
What some people call "narrative filler" others call playing the game. I've never heard Vault of the Drow described as "narrative filler until Lolth is killed"! Of course, if your narrative (or, if you're playing, that of your GM) is boring and creates no room for meaningful player choices, then my commiserations. Those of us who are using skill challenges successfully have mastered the arts of avoiding boring narrative and avoiding railroading.
I call it narrative filler because ultimately the choices, plans, decisions,setting, etc. beyond using something to garner one more success or failure... have no bearing on your success or failure for SC's... it comes down to whether you roll x successes or Y failures that's it. I am not speaking of boring narrative or railroading (though deciding beforehand that the player must get X successes before Y failures before they even approach the situation does seem... a bit linear.). I am speaking of narrative filler in the sense that what you narrate as a player doesn't affect anything about the SC or achieving your goal.
Another issue here, that is hard to address in the absence of concrete examples, is the relationship between scene-framing and action resolution. If the "brilliant plan" is one which kills off the scene as framed, then of course the challenge doesn't continue. If the challenge, for example, is negotiating with the Duke, and in the course of the challenge a PC activates his/her Horn of Invoked Devastation (or whatever) then obviously the negotiation does not continue. The scene has been peremptorily reframed.
The impression that the 4e SC rules left me with was that the use of a magic item would be the same as garnering a single success for the SC...Is the above just how you would handle it or is something like this addressed in the rules somewhere for 4e's SC's?
You said upthread that skill challenges "have a DM create a limited and artificial construct". I take it that you are accepting my drawing of a comparison between skill challenges and extended contest mechanics in other systems, and are now saying the same thing is true of HeroWars/Quest.
No, they are different... as just one example take this passage from Heroquest...
"If the action attempts to leapfrog a series of interesting
obstacles to solve the main problem of the story in a
disappointingly abrupt fashion, assign a Very High Resistance.
(If they succeed, you must then find a new main problem
arising from their solution of the one you were prepared for.)"
So even in Heroquest you can come up with something brilliant and roll high enough to basically auto-success an extended challenge... Is there anything like this in 4e's rules?
I hope you can appreciate that (as you yourself put it) others' mileage might vary - of the various criticisims I've read of HeroWars/Quest, "limited", "artificial" and "boring" are not that common. Presumably you are aware that many people regard it as a hugely innovative and powerful system. And therefore, I would hope, are able to see why some people might regard skill challenges as comparably effective as a mechanic (obviously not as innovative, being highly derivative, and also having maths issues that arise from the extreme scaling of the 4e maths, but equally having some nice features that not all extended contest systems have).
Hey I have no issue if you enjoy SC's... but that also doesn't mean I'm not going to say what I think are problems with the mechanic. As far as Heroquest goes... I honestly see very little discussion of it at all on most rpg forums, but I'll take your word that it is enjoyed by many... it just personally wasn't me or my groups cup of tea. That said Heroquest's rules are, IMO, much better for this type of play than 4e's rules are and I think sometimes, because you've read and played Heroquest... you fill in the gaps and problems of 4e's SC mechanics with stuff that isn't really part of the 4e rules or advice...