I guess people would like to know software which is better and newer than available for 68k, now when it's possible to run on WinUAE.
I haven't used OS4 that much, but I'd say that for example Odyssey Web Browser and MUI-mplayer are pretty cool compared to what's available for OS3.x.
Go browsing http://os4depot.net/
for categories you want... many 68k programs have newer PPC native versions etc. Few native demos etc... maybe Sketchblock would be worth try? Never tried myself though.. I could give more killer apps for MorphOS, but we'll have to wait more if it's ever seen under emulation
Cherry Darling's games are quite cool too: http://www.cherry-darling.net/