MorphOS can be configured to be very OS3.x-like, and that's how I've got my system configured. The only thing I really miss is that not all of the traditional menu options are available in the pull down menus. They are in the context menus, though.
Spent a long time tweaking and changing things but couldn't quite get it to feel like an Amiga. The butterfly doesn't help, miss my boing ball ;-)
The bones of MorphOS is good however, hardware support is better than AmigaOS 4.X. The developers are quicker to release updates and work on driver support. Heck even my 9800pro card works with it on my Pegasos II board.
Warp3d on AmigaOS 4.X is starting to make some improvements but they have side stepped some older hardware, come on support a the 9800pro!!
Hoping for some OS3 love with the warp drivers so I can finally ditch my Voodoo 5 and use the 9200 for more than just a RAM card.
Anyway back OT. Don't go changing the GUI :-)