Raze already stated in at least on other thread that the UI will be changed/tweaked once the underlying mechanics have been "finalised".
A lot of what you see is placeholder, it's just relatively polished placeholder and as such it's easy to assume that that is final.
It has also been stated that there will be graphical enhancements before release as well.
As for what makes BG BG stylistically, well the Infinity Engine approach has a lot to do with it. Painted assets giving a false painted reality look. Muted colours, though not necessarily dark or without colour just more of a matte filter.
3D modelled games have always been shinier. Even Witcher 3 is vibrant and full of colour for such a dour story.
That said ignoring placeholder assets in UI and world, the main characters showcased so far look a little too "new". Like thy stepped out of a comic con. Witcher the Netflix TV series was given the same criticism, costumes and style looked too much like a tv set and less like a lived in world.
Spiderman on the PS4 - Vibrant (it's super heroes!), but you start off in this worn out suit and you work at getting it patched up or replaced. It's a nice touch but it meade me feel like I was playing a Spiderman that had already been around all the blocks multiple times.
Here, in the Demo and I stress DEMO, the main character (Astarion) comes out of a crash looking pristine and Shadowheart already has back what looks like all her gear. Now we didn't see the tutorial, so we comment on this DEMO scene as being the "opening". But if we have just been through (proverbial) Hell, like Shadowheart puts it, make it look that way, because BG never felt clean, even though it is still High Fantasy.
It feels old, from the stone UI to dirty boots and rusty swords and whilst colour is good, sun is good ( I don't need constant grimdark), different settings are good, lived in is better. Does that make sense?
