There is a subtle but interesting distinction between Dcs and "roll under" that i think is under appreciated. It revolves around "what defines difficulty" in an external vs internal locus of control sort of way, respectively. In the DC mindset, your stat has a small impact on your chance of success, but the actual difficulty of that thing is based on what IT is (either in the narrative or the simulated world, but that is a different issue). With "roll under stat" difficulty is nearly entirely built around ones capabilities. All climbs are equally difficult (within some small range of modifiers); what really matters is the natural talent of the climber.
From a game design and theory standpoint, I always thought this was an interesting question and debate. As stated, I tend to fall on the side of DCs -- that is, the simulated reality of the thing defining how difficult it is.
EXCEPT, I like old school saving throws better, where the ability to make a saving throw is almost entirely defined by the capacity of the character. Saving throws are sorts of measures of heroism, so it makes sense to me that they are "internal."
Of course, this has almost nothing to do with Shadowdark. Your post just got me thinking about it.
