Diablo may 'only' have 24 frames of animation, but you can bet that they're scrolling the background at the same rate as the refresh rate to get it nice and silky smooth.
The background moves as fast as the characters: 25 FPS. This is the developers ideal setting. As many of you know who have played Diablo II keeping even at that consistant frame rate would have been nice
Or play Bauldurs Gate 2 and compare how smooth the characters are (traditionally animated sprites) against the spell effects (particle & vector based animation).
The big difference there could be that Baldurs Gate has over 108,420 frames of character and monster animations. Part II saw and addition 100,000+ frames (see games technical faq). The more unique animation frames you have the more 'silky' the end result can be.
For 2D, not only framerate, but also constancy of framerate is highly important,
Can't agree more. Diablo could possibly run at 200 fps on a good computer but the designers purposely cut this down to a self imposed 25 FPS. This is because movement and animation in games is often based on the desired frames per second. Its all about timing. This also prevents the game becoming too fast in the future when you have better hardware. Think of them as self imposed speed caps.