I think this is a very important point and perhaps one worthy of its own thread. The best hand isnt the hand which would win the pot if no more cards were delt. The best hand is the hand with the highest chance of winning the pot at the showdown.
If you are heads up with 0 money in the pot and a guy with face up cards goes all in for $100 with JdQd on a 9d Td 2s board and you hold Ac Th you may have the highest pair at the moment but you only have a 31% chance of winning the hand. Calling just because you have the "best hand" at the moment is a crazy play and will lose you alot of money in the long run. In holdem this doesnt happen very often. In Omaha it is crucial.