To me, easily the most compelling feature is multiple environment management.

I am constantly bitten by this, say, whenever I want to run multiple Ruby/Rails apps on the same machine.

Experiencing "can't activate foo (= 2.0, runtime), already activated foo-1.0" errors is a terrible way to start the work week.

I'd like to hear more about how it resolves dependency conflicts more conveniently than Rubygems though.