[Reedx]

Right, DotA was a mod no one asked for.

Portal was actually a similar thing. It started as a student game called Narbacular Drop. Valve hired that team, who turned the concept into Portal.

Likewise they hired Icefrog from DotA to work on DotA 2.

That's a good strategy.

[mos_basik]

It's an excellent strategy. To add to your examples: Team Fortress was originally a Quake mod, and Valve hired its creators to head Team Fortress 2's development. Counter-Strike was originally a Half-Life mod before Valve bought the rights and made CS:S, then CS:GO.