Video playback is one major task that I'd have thought a 68k CPU would struggle with (I believe that MPlayer is only avaliable on OS4 and MOS as well). And even audio playback - of course a fast 68k chip can play back MP3s fine, but it'll use up a much higher proportion of CPU power than it would on a faster system. Also, it Amizilla gets finished, don't expect it to fly on 50Mhz :-P
As for native PPC software - there isn't much yet (although JIT emulation on PPC might beat your processor for raw performance, I'd imagine). There are some fairly significant projects in the pipeline that will be native PPC though - like RealSoft 3D for OS4. You can bet that all ports of 3D games will require PPC as well.
