The rolling, sticking, never-stopping, ever-swelling clump of stuff that makes a star of out everyone and everything.
Namco's Katamari Damacy is one of the most original and charming games that I've ever played. During the game, you roll around a sphere around various places. Smaller objects, like tacks and matches, stick to it, bigger things don't. Yet your katamari grows as things stick to it, and the once unassailable blockades become fodder for your ever-snowballing pile of junk.
The initial enaging element is, haha, look what I just rolled over and squished onto my ball. The secondary element of the continuous increasing of scale is surprisingly and lastingly engaging. Often you'll start a level rolling over tacks, rolling around the heels of towering humans. After a minute or two, you're knee high to them, then soon enough you can knock little children over and consume them! Then cars! Then buildings! The epiphany that occurs when you realize that what once was a limitation is now an asset is a great sensation.
It is hard to discuss Katamari Damacy without continually praising the insanely catchy music. The wide array of musical styles all imbue the game with a sense of playful wonder that frames the innocent element of the player's all-consuming mayhem.
I recommend Katamari Damacy to anyone who thinks rolling over a bunch of little girls is even slightly funny.
postscript: Penny Arcade put up one of their funniest comics about the game. Good stuff.