As identified by EliotJ. Also marked in bugzilla with the regression keyword:
Original notes:
Bug 362136 - Instant preview doesn't work for the spray brush (Imho this can be treated like regression, from user point of view. Because “Instant Preview” is enabled by default in 3.0 which means users can see this bug directly after migration to 3.0. Simply default settings on 2.9 doesn’t give this bug. I think average user simply downloading Krita and start painting, not thinking what is changed lately under the hood, but he noticed if something going to be wrong now...).
Bug 361709 - With more than 1 curve based assistant, zooming in will black screen part or all of the drawing window (It looks like OpenGL rewrite related).
Bug 362470 - [WACOM] Cursor offset problem (I know there is duplicate bug of this, but in comments of this is a confirmation that it’s a regression).
Bug 362357 - Basic words like OK, Cancel remains English when running Krita in Japanese.
Bug 362046 - Boost fails to compile (If I understand it right it’s a regression).
Bug 361448 - Brush strokes flicker when they finish rendering (Imho this can be treated like regression, from user point of view. Because “Instant Preview” is enabled by default in 3.0).
Bug 361550 - The file dialog has light icon theme making it hard to see them (related bug report - Bug 361549
Bug 362712 - Crash when switching OpenGL on/off.
Bug 361653 - Show Painting Assistants has no effect.
Bug 361734 - krita does not support OpenGL 2.1 anymore.
Bug 362629 - Certain Brushes do not behave the same from 2.9 to 3.0.
Bug 362600 - Incorrect hue values of the colours displayed by Krita (splashscreen/interface/canvas) while using mesa.
Bug 362032 - [OPENGL] krita won't show menu dropdown in fullscreen mode on 3.0 alpha git f38b47e.
Bug 361585 - Two-finger gesture to zoom on trackpad causes zoom-mode to be always on.