To be fair all of the more sophisticated systems require a lot of fiddling to use the whole software library (Macintosh, ST, IBM PC), but I guess I am just attracted to the simplicity of the C64 where a stock unit with a disk or tape drive can run the entire software library so long as a dongle wasn't required. No configuration, etc. From a collector stand point, extremely accessible.
The Amiga is pretty tough for folks who never owned one. Even as a long time Amiga owner, I constantly rediscover things I knew about AmigaDOS back in the day that I had since forgotten.
The biggest plus to Amiga collecting is some superb box art I suppose.