Diablo – Movement speed 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

Diablo III is not sprite-based like Diablo II was (Instead, it's full 3d), so there are no longer discrete frames.

The gameplay ramifications of this, of course, is that any amount of increased attack speed is equally viable. (Though not necessarily noticeable)

You can view your current attacks per second on your character sheet, and unlike in Diablo II (which listed only a speed, e.g. Slow / Fast / Very Fast), you'll actually get a decimal value.