http://www.sandboxie.com/index.php?FAQ_Licensing

Q. Which features are available in the paid version? A. Two features: • "Force" programs: Automatically run programs under Sandboxie even when they are not started directly through Sandboxie. Programs can be "forced" by name or by containing folder. • Run programs in more than one sandbox at the same time. This causes error SBIE1303 in the free version.

The first one is very useful (a.k.a. set up Sandboxie in a way that BFUs won’t have to deal with sandbox details and yet won’t be able to infect the machine by browsing pr0n and warez sites).