Gotta catch 'em all
My children are wholly obsessed with Pokemon.
I dunno why, but a few months ago I totally gave up any pretense of trying to control their access to their Game Boy Advances. What they play almost to the exclusion of everything else are Pokemon games. They go running full speed back and forth to wherever the other is in the house, telling them about their latest exploits -- "I captured this Pokemon" or "This one is evolving" or "I'm visiting a new area" or "How do you get this, that or the other thing."
If your familiarity with Pokemon ends at the cartoon or the collectible card game, the Game Boy Advance game is actually a Role Playing Game (RPG) in which you assume the role of a Pokemon trainer. The Pokemon you control must battle others, and battling them strengthens them, eventually causing them to evolve into different forms.
Each game has dozens of different Pokemon to collect; different areas to explore; different goals; different characters. But at their core, they're fundamentally similar experiences. Yet our kids are determined to play them all.
I've played Pokemon games. Personally, I find them excrutiatingly boring. But then again, I'm not the world's biggest RPG fan either.