The graphics for the character models, environment, etc, are all improved, the camera system has been updated (some of which is in progress).
Right now, it just looks like DOS2. And if "BG3" hadn't been announced and you showed it to someone who'd played DOS2, they'd immediately think it's DOS2.
The goofy movement and action animations, the mouse cursors, the text font, are all directly unmodified DOS2's. And the UI kept DOS2's style and just moved things around a bit. Either all of this is placeholder, or Larian is making a specific very strong effort to make everything about "BG3" make people think of DOS2, in order to make people buy it based on thinking of it as more DOS2 content, or to get people who buy "BG3" to possibly buy DOS2 because it looks the same.
Either way, "BG3" does not look like its own game, it visually looks very much like DOS2. And I don't find that to be a positive thing because it emphasizes that "BG3" is DOS2 in D&D and a cash-grab exploiting the name while not delivering on what it represents (which is supposed to be itself and not DOS).