Off the top of my head, another big one is that Arc has a more library functions. Individual Scheme implementations tend to, but the Scheme standard is pretty small.
Not anymore with R6RS---it has over 650 functions. Besides, I tend to think SRFIs as a part of 'modern' Scheme, as some of them are quite essential for day-to-day programming. (Although the popularity varies a lot---cf. http://practical-scheme.net/wiliki/schemexref.cgi?SRFI )
