Basically, I start out saying that it is frontend, backend and database (i.e. both what you see on your computer and what is going on at "the server" as well as the database), that it is all written in JavaScript (of course many people confuse JavaScript with Java so this can sidetrack a bit), and that it is realtime (i.e. if I update something on my computer and you go to the same site, you can see that update right away.)
Also, I might state that it is cool how you can easily deploy a test site (via the 'meteor deploy' command) and that I have found it to be one of the easiest ways to get a location-based program working thanks to the previously-mentioned easy deployment and its integration with MongoDB which offers spatial indexing.
Alternatively, you can look at how Meteor explains it themselves: http://www.meteor.com/
I always use their marketing speak: "A better way to build apps" but of course that's difficult to back up with brevity and clarity. I find myself going through bullet point features, as it sounds like you do to.