Diablo – way to hot key skills in Diablo 3

You can change skills whenever you want, but there are currently two penalties:

There's a small cool-down in which you won't be able to use the new skill, which makes swapping skills frequently in combat disadvantageous...

...And swapping skills will cause you to lose the "Nephalem Valor" buff that characters over level 60 get from killing elite monsters, which increases your magic find, causes boss monsters to drop additional items, and, since paragon levels were added in patch 1.0.4, also gives bonus experience.

See also the question on Nephalem Valor

No, that feature was intentionally taken out by the designers because they wanted to avoid that situation we all know in D2 where we spend about 10% of our time looking at the action and 90% looking at the minimap.

Source:

You are very much correct in your statement that this was a deliberate design choice, Caelestis. We did not want to have the Diablo II scenario where people just stared at a dot on a map to guide themselves through an area. We made the map full screen, completely opaque and prevented commands from being sent while it is open to stop this from occurring. We also like the concept that your character has to take a few moments and stop so that they can get their bearings :)

As a bonus, you can always kind of see what areas you haven't explored yet in the minimap to the upper right of the screen.