Gee, if OS X is slow on a 700 MHz G4 then I must be missing out on something! I've found the speend of 10.3 is acceptable on a 1 GHz G4. Perhaps not stellar, particularly when a few applications are competing for memory, but a lot faster than what I've used in the past (unless you are using a single measure like boot speed, where something like a Mac 128k could boot and load an application in about 15 seconds).
I can see what your talking about though. From the perspective of disk and memory usage, the current situation is horrendous. There are several many things which I can do on my old 68k Mac which would start thrashing the G4's hard disk, even though the G4 has about 8 times the memory and I've been impressed with OS X's virtual memory subsystem for the most part.
My main concern is that switching platforms will not solve the problem. No offence is directed to the Amiga OS or MorphOS development teams (because I have not seen their products), but it seems as though very few programmers have enough pride to develop small and fast code these days. I also have concerns that too much attention is being placed on the GUI, and too little is being paid to getting real applications on the system. Who really cares if the GUI is fully configurable if you don't have a reasonably comprehensive web browser and productivity applications running PPC native.
If anybody wants to dispel those concerns, feel free to. :-) I would love it if somebody proved me wrong because the current pile of bloatware, which makes a 1 GHz G4 look insignificantly faster than a 68k for most applications, is really turning me off from computers. :pissed: