I do agree that the Snow Queen's motivation as revealed in this episode is a little underwhelming. Most of the threats they've faced so far have been a lot bigger, and have at least been a threat to Storybrook as a whole. Hopefully there will be more to it by the end of the season.
She's going to make all of Storybrooke turn on each other, presumably killing everyone except Emma and Elsa in the process - her motivation may be small, but she's still a big threat to the town.