I think it's awesome you're even having a party! I've never heard of anyone having a party to celebrate. Its not that masters degrees are not important, but few people attend the ceremony and such... you know? Just with my friends/family/classmates... unlike undergrad in which we had a party and more people came. Of course, high school takes the cake for having a big blowout =)
When I graduate with my masters, we'll just go out to dinner or lunch to celebrate, and I would only expect close family to attend. If I was having a party, I would do it more formal just becuase I like that =) I dont think people will be expecting anything in particular, just because you dont attend grad school graduation celebrations that often... like a wedding, people have expectations. For that, they wouldnt, so I'd do whatever you wanted! I think hosting a more formal event would be awesome to plan, but thats my personality and I rarely get to do formal things =) Go with what fits your personality the best! And have fun!!!!!
You've inspired me to maybe hosting a party anyways and hoping people will drive and come. I worked alot harder for this degree than my first, dadgumit!!!!!! =)