The usability of the new interface is really low, sadly.
I have now read the first 8 pages which show that the almost everybody agrees on the same points.
I think we can distinguish between two main areas:
1) New art style
2) New user interface
1) is certainly subjective, so there is no real need to talk about.
2) has been consisting of objective criticism.
Here are my main points about the UI:
1) New clue giving
Yes, less mouse movement now. But now you need to read text (objectively worse than old giant symbols). You also need more clicks (e.g. when doing rainbow). Selecting the correct option from the pop-up window is very error prone (objectively). It is made worse by the fact that the window pops up in front of the hand I want to give a clue to. So I cannot double check what I want to clue while looking at my options and the cards.
2) New visualisation of errors
Its is now a board behind the stacks of played cards. Very hard to see as most of it is covered. Also, dark red next to dark blue is very low contrast, so objectively difficult to distinguish.
3) Discard pile
The old discard pile was real cards. Very easy to see and distinguish. The new one has extremely small numbers in very low contrast colors. It's near impossible to see the purple and the black on the dark background. Small size and bad contrast are again objectively bad. Some people also have problems seeing everything without scrolling.
4) Player colors and turn marker
Very difficult to see some player names (I think this has been partially fixed). The big red rectangle below a player when it is their turn is very... ugly? It does not fit the graphic style of the rest of the interface. Also objective problems.
5) Playing or discarding
I always use the buttons at the top. But you can also do drag&drop. So having both is great. In the old version, you would get a green highlight around the "play" and "discard" areas to show you that you can do this. The new version does not have it - so many people might not figure this out.
Some general comments about the redesign:
One argument I have read a few times was: "This is how the game is played around a real table".
This is true for the new (optional) arrangement.
But it is not true for a lot of other elements:
a) Given clues about my cards are displayed on the cards. In the real game, I need to remember this.
b) The real game allows me to sort my cards.
c) In the real game, it would be impossible to show the errors like in the current version. As the board is underneath the "played" stacks. This would mean lifting them and putting a new tile underneath.
d) The discard pile is totally abstracted as no cards are used but just numbers.
Therefore, I do not think the "that's how it is in reality" is a particularly strong argument as there is some picking&choosing going on. Hanabi on BGA should either be as faithful as possible to the "real table" version. Or optimised for screen-based play. Atm, it is objectively a mix of the two.
It is clear that a lot of hard work has been put into this new version. I don't think anybody is denying it. It just feels that usability was not one of the main concerns. And this is arguably the most important aspect next to bug-free.