In my opinion, after seeing as many screenshots and videos as possible, the only area where I can argue CDPR actually scaled back the tech for tech reasons is maybe the foliage. Pretty much every other visual difference I've seen between 2014 and now has been an artistic decision, and that'as AFTER you ignore the dubious comparisons.
Most of the comparison screenshots I've seen compare one slightly compressed screenshot to a heavily artifacted YouTube capture, or compare two shots in the same location but at completely different times of day. Just the addition, removal, or change of a single light source can completely change a scene to the point where a 1:1 comparison is impossible.
Aside from that, the one really obvious change you can see between 2014 and now is that sometime late last year CDPR got rid of the sharpening filter, which was a very unpopular feature of Witcher 2. That alone gives Witcher 3 a much softer, almost painting-lite look. I consider that a purely artistic choice, and a matter of preference. Some people seem to prefer the heavily sharpened look. Aside from that CDPR probably also did a lot to the color balance. The shot of Novigrad from the E3 2014 gameplay reveal trailer almost looks like it has a green filter all over it.
I'm also gonna throw this observation out there: The gameplay reveal, VGX, and Sword of Destiny trailers are all mostly made up of extremely short, likely heavily cherry-picked shots. Like half the shots were of real-time cut scenes too. The Elder Blood trailer is ALL real time cut scenes. We have no way of knowing what kinds of flaws the actual gameplay graphics had back then. Judging by the original 2013 screenshots it probably looked very similar to how it does today.
Lastly, I think a guy from CDRP, maybe during a Gamescom or something a while back, let it slip that the old builds we saw around E3 2014 were running on a Titan.
In any case, the changes we've seen here are nothing like Dark Souls 2, which undeniably had an entire dynamic lighting system removed from it (and that was one of my most hyped games of last year).