Out of curiosity, how do you view 4E treating so many non-combat; non-skill challenge situations as DM Fiat? (i.e. wanting to build a castle)
That's not DM Fiat though.
There's a difference between something being rules light (ie, how skills or non-skills and roleplaying worked in all editions other then 3e) and requiring the DM to change the rules (ie, monks in 3e).
Rules-light means the rules cover what needs to be covered and leaves things that don't require rules up to the group - and it's more up to the group then it is just up to the DM.