Easy to use and very intuitive interface, fast and stable, collaborative. Unlimited workspace with custom frames is just awesome! :)
Not so many standard shapes.
I'm not a designer, but I use RTB a lot to create simple sketches for some of my ideas on our products with comprehensive text descriptions. One image worths of thousand words! My sketches helps our designers and developers to quickly understand what I'm talking about and how my ideas connect to other products (thanks to unlimited workspace). I used to use Google Presentations for these purposes before, but RTB just stolen my heart! Good luck to the RTB team! Great product deserves fast organic growth! :)