The most useful aspect in my opinion is the ability to basically 'drag and drop' geometry to assemble a form as shown in the series of 'quests' used to teach people how to get started with Tinkercad.  This would be even more powerful if one could import an STL like in Meshmixer to modify more complex forms such as an iPhone case or mouse housing.

A beneficial aspect of a browser based modeling program is that Tinkercad can continuously update and improve the software without the need for users to download updates. So along with these major changes to the interface and functionality, Tinkercad can continuously improve the user experience.