What did you think looked kiddie and ended up being cool?
A perfect example for me would be Grand Theft Auto 3. I remember seeing the box cover and thinking it looked too cartoonish, or "for kids" because I thought the cartoony style of the cover art represented the look and feel of the game itself. Needless to say, it was WAY cooler than most people thought it was going to be. Other examples would be;
Animal Crossing - http://www.animal-crossing.com/ - suprisingly addictive, even if you're normally into war games n such
Munch?s Oddysee - http://www.oddworld.com/ow_frameset.html - great game, no matter what age or what type of gamer you are. Probably my favorite platformer ever.
I-Ninja - http://www.ininjagame.com/ - took a chance on this one and I'm so glad I did. Another great platformer with some of the most fun moves in a platformer.