LEGO Minifigures Online looks to be a combination of the LEGO adventure-style games you're no doubt already familiar with, along with the ability to collect and even improve a bunch of different characters. Finding stars earns you more bags, more bags means more figures, and more figures means more abilities. Plus it seems like a pretty solid reason to keep playing.
You'll be able to get your hands on LEGO Minifigures Online later this summer. Although if you're going to be in San Francisco on Tuesday, March 18 - Thursday, March 20 (i.e. during GDC), you can also try to swing by The Westin San Francisco Hotel for a live demo, or possibly some hands-on time.