One of the issues in the past with Microsoft Office SharePoint Server (MOSS) 2007 was controlling the use of SharePoint Designer (SPD) within SharePoint's Central Administration or within a site collection, or even granular control at the site level. Well, it appears Microsoft has listened to the voice of the customer and implemented a solution which allows an administrator to check a box and limit SharePoint Designer use on a site collection basis. With some companies creating most sites as it's own site collection, this is a great option.
Now, I know there were ways by modifying web.config files and other work-arounds to allow this similar feature but in most cases the feature had to be implemented on a site-by-site basis or it was "all-or-nothing". Adding this feature will allow enterprises and administrators decide how and who is allowed to make design, workflow or other modifications via SPD 2010.
Some of the other choices are enabling the detaching of pages from the site definition, enabling customized Master and Layout Pages, and enabling the management of the web site URL structure. These choices are self-explanatory, but have great consequences of how much a user can change on the site. Options are always best in this case.
Based on previous deployments, I know this ability will be very well received because some companies what users to have no ability to use SPD and others allow the full use of SPD, it's a governance choice. With the new interface, there appears to be more ways that a site can be modified and SPD 2010 will only be able to modify SharePoint 2010 sites so only the future platform will be impacted.