MorphOS has multi-select menus too.
I don't see how MUI makes people put too many buttons in the programs. Do you mean because it's very easy to make GUIs with MUI compared to using Gadtools or Reaction? Sure, it's a lot easier for the programmer, so we can get on with coding the important parts of the program and not have to worry about spending too much time programming the GUI for it. It has been like this since the early 90s when most Amiga programmers started using MUI for their programs, and MUI doesn't have any unnecessary eye candy, that's purely up to the user if they want to make their programs look visually appealing insted of keeping the grey, default look. Even the GUI in OWB is based on IBrowse, other MorphOS software is extremely Amiga-like. And once Zune is updated, Aros software will have the same amount of visual configurability as MUI4 in MorphOS. The main thing missing at the moment is the ability to use bitmap frames rather than the default MUI vector ones.
MorphOS can use regular Amiga icons, GlowIcons, NewIcons or PNG icons and they can be single or dual-state, up to 32bit.