The e-commerce functionality, available in the new Ultimate and the existing Small Business editions, adds payment and user account features. This makes the creation and management of a storefront significantly easier, and does a nice job of rounding out Site Manager's feature set.
Both new editions feature beefed-up platforms for building sites with resilience under high traffic loads. Along the same lines, support for Oracle and MS SQL databases has been added to the existing MySQL support. That's likely to broaden Site Manager's appeal.
Site Manager has built a substantial customer base by offering feature-rich products at a value-minded price. While it isn't open source, it shares some flexibility with those platforms because the source code is included with the license of the Premium and Ultimate editions. It's certainly worth a look for organizations that need to build a site with a flexible, solid framework on a budget.