When most people build lists, they go for a very mixed and balanced approach. However, most codexes have the potential to throw enough bodies or armor at your opponent to frustrate him beyond belief. I am, of course, talking about the Unbalanced List.
The Unbalanced List is not necessary an overpowered or broken one, simply one that aims to maximize a certain strength in order to completely minimize or make ineffective the impact of a balanced list.
By removing one or more elements entirely from your list and using those points to further enhance another element, you create the unbalanced list. Examples are the Imperial Guard Armored Company (a couple infantry squads, command squad - all in Chimeras, and then more Leman Russes than you can shake a stick at.), the Ork Horde, Nidzilla, Elfzilla, IG Infantry spam, Dreadnought/Kan spam, and so on so forth.
The idea is to completely overwhelm your opponent's ability to nullify your strong point (Kill their templates, anti-armor weapons or fast movers) and then just clean up everything else. The beautiful thing is that even it takes you two or three turns to wipe out your opponent's anti-armor (and you lose one or two pieces in the process), you still have enough armor on the board to be a credible threat that they might not even be able to deal with effectively.
What are your thoughts on the Unbalanced List?