Many users are familiar with XnView Classic and its user interface. Also, the user interface of XnView Classic has been discussed a lot.

An example:
In image view mode, the buttons for navigation and zooming are located in the middle in XnView Classic. This is good because these buttons are often used and can be reached easily. In XnView MP these buttons are on the very right and thus are not reachable that well.
Note: This is not a real bug. But differences in the user interface of XnView Classic and XnView MP are irritating and should be avoided whenever possible This is why I'm posting this as a bug.