Collaborative Storytelling. That sums it up in two words.
A bit more, the following sums up my view on the ideal of an RPG. It will conflict greatly with some other people's views on the game. It doesn't make theirs' wrong (unless I rise to power as supreme overlord... muahahaha...) but this is how I view the purpose and focus of an RPG. This will upset some people, greatly, and it's perhaps lush with stereotypes, so be forewarned and please take no offense, or rather feel free to knock my view around a bit.
It's about character development, not 'but I can get an AoO because he's in this square'
It's about a slowly developing world and a plot in that world which doesn't revolve strictly around the PCs by default, but which they interact with and can influence, perhaps heavily influence. The PCs don't exist in a vacuum.
It's about living, growing personalities of characters you act out, not counting stats on a character sheet to munch them.
It's about exploring a world and developing a character and their place in that world and that context, not about killing things and taking their stuff.
It's a group of characters who have reason to work with one another for mutual, or at least compatable goals, not 'I have no idea who these people are or why I'm here working with them, but we're the PCs and we're the heroes so I'll do this anyways!'
It's about a sense of awe and wonder at a mutual creation between players and a DM, arbitrated by the DM and to a lesser extent the rules as written.
The moment the rules of the 'game' impinge upon creativity, the rules go out the window without a second thought. Rules lawyers are things that wither in sunlight and don't cast reflections. They're not even human.
Miniatures are for playing monopoly with, and for 8 year olds, and basement dwelling 50 year old manchilds to collect along with Yu-Gi Oh cards, etc.
Beer and Pretzels are for bars. Diet Code Red, espresso, and oreos are the true gamers feast.