IMO, it would be more like "well, these are paragon PCs, so climbing the mountain in the blizzard has no check."
If I wanted it to carry said DC of 28, I'd probably make it something like "climbing the mountain in a blizzard during an avalanche" or "climbing the mountain in a blizzard while dodging the attacks of spear throwing yeti at the top of the mountain"... though I think going with the latter I'd trump up the variables even more and go full on Skill Challenge to make things interesting.
Well, I'm getting sloppy. Of course you probably wouldn't have a DC for some major piece of an adventure. You'd have something like an SC, during the process of which you might meet the Yeti, get caught in an avalanche, need to reach the summit before nightfall to avoid exposure, etc. I'd note that in my conception of SCs there's nothing wrong with 'do or die', every straight-up combat is a do-or-die situation, you win or you die (at the very least there are dire consequences for failure). For whatever inexplicable reason the 4e guidelines on SCs indicated that they should represent less critical situations, but I don't really see a reason to abide by that. It could well be for instance that the Yeti is simply unbeatable in combat, so you better not fail the SC and wind up facing them! Maybe if you do you can turn tail and run, but failure here CAN have the harshest possible consequences.
Anyway, the DCs for whatever happens during the challenge? I'd say they can be set as desired, but like I keep saying, narrative consistency still indicates that if something is a 'paragon challenge' then it has to have an overall difficulty that matches. One of the problems I see with the 'objective DC' thing is, you don't really set the overall difficulty of an endeavor by one DC. 4e's DCs are intended to be strung together in a long series. There was never any intention that there would need to be a single very hard to surmount DC that the party would check against, except maybe in some very special situation (IE maybe you get one last chance to survive being lost at night in the blizzard on the mountain, make a hard Endurance check and you don't turn popsicle even after the SC was lost).
I hadn't really considered listing the 5e ritual system as one of the things I don't like about 5e, but then that's probably because it's such a non-entity that it escaped my consideration.
I don't really care one way or the other if it impacts balance. However, it's just complete weaksauce to me. Don't you have to know a spell before you can cast it as a ritual? I guess for some class maybe this is no big deal, but for others, wouldn't it severely cut into your spells known to try to accommodate spells that would be good candidates to use as rituals?
I can get behind the desire to merge traditional casting and ritual casting into a consolidated system, but it just feels like an uninspired tack on to me, unless I've grossly misunderstood how it works.
Well, non-casters can gain access to ritual casting, so it CAN in some sense fill the role that 4e gave it, being a way to have a limited form of magic without a caster. You could have something like 'Elric', a guy that doesn't cast spells per-se, he's a fighter, but he does have some quite potent magical resources he can trot out if you give him some time to do it. Not that 4e really emulates Elric exactly any better than other editions do, but the general idea was attainable. Or a battle captain that augments his troops with magic before a fight or during a march.
Its just weird that in 5e this subsystem is much more effective for wizards and clerics. A 4e wizard does get it free, and gets some free rituals, but a warlord can acquire rituals for a pretty minor cost, and since they aren't actual combat spells they can cover a lot of ground that might only be useful to that sort of PC, or that are never useful during actual adventuring time.
