Myst Online vs Dwarf Fortress.
One is an utterly beautiful and simple adventure game set in an online environment that promotes teamwork, and is played from a third/first person perspective and is utterly relaxing and pleasant, and fun in a much more laid back way than normal. Everything has been planned and designed a specific way. It's light-hearted with some dark moments. Its story is complex and was built up over several years, and the overarching mythos was developed (and epically retconned several times) over the various Myst games. It's entirely non-violent. Dying is impossible. It's absolutely awesome.
http://kotaku.com/assets/resources/2008/02/myst_online_rip.jpg
VERSUS:
An ugly and ridiculously complex God game that's entirely single player, and pits you against everyone else (including, usually, your own dwarves). It's played from a bird's eye perspective with z-slices (yes, z-slices) and is frantic, panicky and all the fun is derived normally out of how hard you fail. It's completely random and unpredictable. It's really dark with some utterly hilarious moments. The story is generated three minutes before you start. It's violent to the point of being simultaneously vomit-inducing and hilarious. The general consensus is that dying is the POINT OF THE GAME. It's absolutely awesome.
<img width=450>http://www.adayinthelifeof.nl/wp-content/uploads/2011/03/df.jpg
I defy you to find two games more different. The ONLY way that they're similar is that their control schemes are both functional yet rather irritating (although Dwarf Fortress is markedly more so).